By Joel Achenbach

Sunday, April 19, 1998; Page W10

The discovery and exploration of a

crater left by an asteroid that may

have killed off the dinosaurs is one

of the most remarkable scientific

detective stories of our time.

Now hollywood is releasing two

movies that suggest humans could

be next. What lies behind the idea

of an asteroid apocalypse?

There is only one road north from Belize City. The pavement has no center dividing line, no shoulders, no railings, and there are hardly any signs. It is just a ribbon of asphalt, winding through marshes and jungles and pastures and great expanses of sugar cane and occasionally blasting through a village. The cars on the highway are outnumbered by trucks hauling mounds of wobbling cane. Sometimes there is ash in the air and the scent of caramel from the burning of the fields.

In the villages people walk beside the road, and children stroll to school, neatly dressed in uniforms, long pants for the boys and pleated skirts for the girls. Some of them live in thatched huts, or in wooden homes raised on pilings with dogs sleeping and chickens scratching underneath, but there are also modern homes of concrete block, a few with naked columns shooting skyward in aspiration of a second floor. Belize has defied the stereotype that a developing country must be chaotic and crowded. The whole country is oddly quiet, peaceful, with fewer than a quarter of a million people, so few that a single phone book easily handles every number. Hit the seek button on a car radio and it might well circumnavigate the spectrum and finally return to the same lonely station. The Information Age hasn't arrived here, and there are only tenuous signs of the Industrial Revolution.

The past is layered upon the present. Queen Elizabeth II stares from the currency, a reminder that this was once called British Honduras. In the cities there are many blacks, descendants of slaves. There are Creoles and Mestizos. There are Garifunas, people of African-Caribbean ancestry. At the borders the inhabitants speak the language of the long-gone Spanish conquistadors. In the mountains there are villages of Maya Indians, descendants of one of the great civilizations of Mesoamerica. Their history is still being scraped from the jungle, temple by temple, stone by stone.

Eventually the Northern Highway reaches Orange Walk. At that point you can veer west, toward Mexico, and navigate the back roads, dodging car-swallowing potholes, until you reach a dark, calm river. A bridge is made of planks stacked on floating barrels. On the other side is Albion Island. There is a famous quarry there, a quarry that is not so much a hole in the ground as a great gouge through the heart of a hill rising high above the surrounding coastal plain. In broiling heat the quarry workers drive bulldozers and massive groaning trucks, hauling rocks toward a conveyor belt to be crushed and ground and turned to gravel.

There are other people who come here to handle the rocks -- geologists. They are struck by something high on the quarry wall. There is an undulating line in the rock face, a distinct boundary between one geological era and another. This is the K/T boundary. Below the line are normal, sedimentary rocks formed in shallow seas over tens of millions of years. Above the line is a tremendous jumble of stuff, a matrix of rock and grit and gravel and stones and, here and there, boulders the size of houses. This thick layer of material, the geologists believe, did not take millions of years to form, but rather a few minutes. This is splatter from a catastrophe. All this rocky debris apparently rained from the sky one terrible day 65 million years ago.

That day a large object from space smashed into Earth. It was roughly seven miles in diameter, and moving at hypersonic speed, maybe 20 miles per second. It was like a giant cannonball, blasting a hole 120 miles wide and 30 miles deep near what is now the Mexican town of Chicxulub. The Chicxulub impact was felt around the world. It delivered doom to about two out of three species on the planet, including the most dominant land animals, the dinosaurs. The dinosaurs had ruled the world for 150 million years, by any measure a successful family of animals, perfectly adapted to the blue-water planet circling a warm and dependable star. They had it made. And they never knew what hit them.

Before you finish reading this sentence an ugly hunk of black rock the size of Mount Everest could bear down on Earth from the gloom of space, pierce the atmosphere and smack the Atlantic Ocean like an anvil dropped on a puddle -- the force equal to a billion Hiroshimas, vaporizing a massive chunk of Earth, hurling boulders to the other side of the globe, spawning fires around the planet and sending tidal waves that will shortly destroy every city along the East Coast and eventually lap at the foot of the Blue Ridge Mountains.

But . . . well, it didn't happen this time. (Even though it was a long sentence.)

In a way, these Doomsday rocks don't really come from space so much as from time, Deep Time. Such a rock has no biology, no natural life cycle, and thus is infinitely patient. It can glide through the near vacuum of space for billions of years until one day by random chance it finds a planet in its path. These impactors are irrefutably real and yet strangely hypothetical -- so buried in Deep Time and so hard to detect with earthly instruments that until recently hardly anyone considered them an important element of Nature.

No deadly object is likely to strike our planet anytime soon, but we are about to be seriously cratered by Hollywood blockbusters. NBC already has given us "Asteroid," recently rebroadcast, featuring a bunch of rocks from space with an uncanny ability to hit skyscrapers (asteroids that can aim!). On May 8, Paramount Pictures will deliver "Deep Impact," about a Chicxulub-size comet on a collision course with Earth. Unless ingenious NASA scientists can deflect the comet, all large creatures on the planet will be wiped out. Soul-searching ensues. Then, on July 1, "Armageddon" arrives, the product of the cheerful minds at Disney. This time the menace is from an asteroid "the size of Texas." The official movie synopsis:

"Its impact would mean the end of mankind. With only 18 days to spare, NASA discovers the 'Global Killer' bearing down on Earth. NASA's director, Dan Truman (Billy Bob Thornton), is left with only one option -- send a crew up to destroy the asteroid. Truman enlists the help of Harry S. Stamper (Bruce Willis), the world's foremost deep core oil driller . . . Stamper and his Dirty Dozen crew must land on the asteroid, drill into its surface and drop a nuclear device into its core . . ."

So there's an element of tension. These planet-whoppers may only come every 100 million years, but for a couple of hours in the movie theater the problem is going to feel urgent, pressing, a clear and immediate hazard. A year ago people were worried about invasion by heartless aliens, now they're worried about mindless rocks.

Of course we're all too sophisticated to be really concerned about the End of the World. We're not superstitious. We're scientifically minded. Which is why everyone got spooked last month when actual scientists from the International Astronomical Union announced that we were facing potential catastrophe. The IAU said a rock the size of -- well, not the size of Texas but definitely the size of Falls Church -- was heading our way and would possibly strike the planet at 1:30 p.m. Eastern time on Thursday, October 26, 2028. The rock, named 1997 XF11, seemed on course to pass 30,000 miles from the center of Earth, close enough for an astronomical high-five. If the estimates were wrong and it passed within about 4,000 miles of the center of Earth, there'd be a serious problem, because it would strike the surface of the planet. A leading asteroid specialist, Jack Hills, told a reporter, "It scares me. It really does." The London Times wrote in its lead story, "Apocalypse could be just 30 years away." Fortunately, the Apocalypse lasted only one news cycle. Astronomers double-checked the rock's orbit and announced that it would actually miss by 600,000 miles. NBC's popular Thursday night lineup would not have to be preempted after all.

So, was it a wake-up call, or just more evidence that scientists are constantly contradicting themselves? How should a rational person think about these things? Should a person worry about low-probability events with extremely severe consequences, like asteroid impacts, or does it make more sense to worry about high-probably events with minor consequences, like getting a parking ticket in the District?

It's a numbers game. Scientists guess that there may be as many as 2,000 asteroids a kilometer in diameter that cross or come near Earth's orbit. Such rocks are large enough to inflict global damage. There could be several hundred thousand smaller objects capable of wiping out entire cities. Most of these rocks, large and small, remain undetected, their trajectories unknown. Some experts argue that a person has a 1-in-5,000 chance over the course of a normal lifetime of being killed by an asteroid or comet impact. There is a greater risk of this, they say, than of dying in an airplane crash. Such estimates presuppose that an impact would kill upwards of a billion people. Duncan Steel, an Australian who has searched for asteroids, argues that a person living in San Francisco has a greater chance of dying from a catastrophic impact (because a big rock in the Pacific would generate a horrific tidal wave) than of being killed in an earthquake along the San Andreas Fault. Planetary scientist and asteroid-finder Tom Gehrels says, "This is the most serious environmental danger facing humanity. It can come on you all at once. Kaboom! And you're out."

The likely warning time for the planet, says NASA scientist David Morrison, would be zero.

"Feel the ground shake," Morrison says. "See the fireball over the horizon."

That's the kind of talk that gets you glancing nervously at the sky. We merrily cruise through space in utter complacency that all is well, and yet we may be heading straight at a giant chunk of rock or ice. Earth is the Titanic.

But this may be the wrong way to look at the situation. The survival reflex -- the instinct that makes us look at everything in the context of how it directly will affect us -- may obscure some underlying truths about these rocks from space. To really understand these asteroids and comets and what they mean to human beings, a telescope is not the most useful instrument. A magnifying glass is better. This is because space bombs are not just an astronomical possibility but a geological fact. By examining the past, including the craters, the ejected rocks and the fossils of extinct species, we can put together a more coherent assessment of our future.

Only in this decade has the phenomenon of impacts been fully appreciated. For 65 million years, the Chicxulub crater sat upon Earth as a slowly eroding scar, waiting for someone to notice it. How it got discovered is a great scientific detective story, one that has spawned controversies and remains full of unsolved mysteries. To learn more about what's known, and see the hard evidence of this crater, I went to Belize.

There may come a day when tourists will make the same trip, to see what's left of the catastrophe. Chicxulub has personal significance for human beings. Our mammalian ancestors had been small nocturnal creatures cowering on a planet terrorized by giant reptiles. Then one day something fell from the sky. Without that freak disaster there might be nothing on the planet even remotely resembling the human race.

The quest of our species is to understand why we are here. The old crater whispers a story we hadn't heard before.

In the town of Corozal, not far from the Mexican border in the north of Belize, I caught up with an expedition of scientists and volunteers from the Planetary Society. They were searching for crater ejecta and hoping to find new sites to study the K/T boundary. Everyone felt exultant -- they were sailing a new paradigm into an exotic location. In a world that seemed already totally mapped, the Chicxulub crater and its outwash are new, large, dramatic features. Voyages of discovery aren't completely over; you may simply need to look at old places with a new vision.

There was no distinct division between the scientists and the volunteers. They all had gear -- sunglasses, water bottles, bandannas, ponchos, gloves, flashlights, binoculars, digital cameras, compasses, knives, hand lenses, handpicks, scrapers, jabbers, old toothbrushes and bottles of acid. They had boots and wide-brimmed hats and vests with lots of pockets. They had lotions, Chap Stick, moleskin, repellents, malaria pills. They were geared up for serious science.

Joe Breeden came from Santa Fe, N.M., where he is a business consultant who deals in chaos theory and genetic algorithms -- he somehow can apply cutting-edge biological principles to the needs of corporate clients. As a high school student he used a three-inch telescope to determine the rotational speed of the sun and its gaseous composition. Sandy Miarecki came from the Mojave Desert, where she is a test pilot at Edwards Air Force Base. She has applied to be an astronaut. Roger Nordin flew in from the village of Skelleftea, in Sweden, where he searches for gold and silver and other precious metals in deep mines. He is a soft-spoken man who knows that, at his normal pace, his steps measure precisely 0.7 meters. From Upstate New York came Paul and Jutta Dudley, whose relationship was solidified by the mutual love of geology.

"Our first date was fossiling. In the Devonian," said Jutta.

Bob Cozzi came from Illinois, where he writes books about software and, lately, campaigns for Congress. Regardless of whether he wins this year, he can still brag that he once found a fossil of a previously unknown species of crab.

From San Diego came computer engineer Tim Herman, who, according to his one-paragraph biographical description in the expedition handout, "is an adventurer, truth seeker, nuclear physicist, rocket scientist and computer guru." He is, in short, a Californian.

Competing with Herman for most colorful character was Bruce Hartel, a big-game hunter from Colorado who has 35 animal heads mounted at home. He'd been to Belize several times, trying to add a jaguar to his collection, back before jaguars were made off-limits. "Man is a predator," he said when someone challenged his love of the kill. Hartel told stories of near-death encounters with raging beasts. "The second cape buffalo I took, I shot him one, two, three times. Every time, he went down and got right back up and charged. I finally dropped him with a frontal heart shot. That put the spice in the dish, that's for sure."

Adriana Ocampo and Kevin Pope, from Los Angeles, were the expedition leaders. Ocampo is a NASA geologist who likes to say, "It's the dream of every child to get to play in the dirt. We get to do it for real." Pope is a kind of Indiana Jones character, a freelance geologist and archaeologist turned Chicxulub expert. In 1988 Pope was in Mexico at a scientific conference, discussing a ring of sinkholes, called cenotes, in northern Yucatan that had been recently detected by satellites. He was interested in them because they were important freshwater sources for Maya civilization. Ocampo heard the talk and approached him afterward. Is it possible, she said, that this ring of sinkholes marks the outline of an impact crater?

She was right. There had been fracturing of bedrock at the crater's edge that allowed groundwater to seep to the surface. The revelation set them off on a scientific adventure, and at one point they paused long enough to get married.

The crater from the Chicxulub impact is no longer easily detected without high-tech imaging equipment. That's why the expedition came to Belize. Models of the impact show that this area should be full of ejecta. The best site found so far is the Albion Island quarry, about 200 miles from the crater. We all drove there one morning, bouncing over the rural roads in rented vans. Pope discovered Albion in the mid-1980s as a good place to swim. He would come and dive in the pond at the edge of the quarry. He had no idea that above him were these rocks from the disaster that wiped out the dinosaurs.

At the quarry Pope briefed the troops. He pointed out the K/T boundary on the far wall. It included a distinctly orange line about one meter thick. He and Ocampo believe everything on top is the ejecta blanket. Three lines of evidence indicate that this is the spew of the Chicxulub crater. First, it's a chaotic mix of material, from giant boulders to tiny blobs of clay. Second, the rocks have markings, various lines and grooves, that suggest they were banged up and scuffed in countless midair collisions after being launched from the crater. Third, there are distinct traces of weathered glass, apparently from rock melted by the intense heat of the impact.

None of this has hardened yet into scientific fact. Pope's dream was to find a shatter cone, a peculiarly pyramidical fragment of rock that has been found at other crater sites. He thought he had a shatter cone already, but Ocampo wasn't convinced it was the real thing. There also remained a serious question as to how long it took for the ejecta to get here -- seconds? minutes? -- and why parts of the ejecta blanket had a layered look, as though neatly laid down.

We divided into teams. One team looked for shatter cones, another looked for fossils. One team drilled holes in the rock and took samples for magnetic analysis -- a technique that can help determine the age of the material. There was much interest in a house-size boulder that had recently been uncovered by a bulldozer. Could it have flown here all the way from the crater?

One might say the essence of science is knowing what you're looking at. It was not enough for the Vikings to land on North America -- they didn't know what they'd found, and so never truly discovered what would later be called the New World.

It took a long time for humans to discover that they lived on an old planet, and that its history was written in the rocks. In the late 1600s people discovered the principle of stratigraphy, in which the layers of sediments formed a chronology of the past. In the early 1800s geologists began defining geological periods according to changes in the fossil record. The rocks told of life and death on Earth. In 1815 they recognized that there was an abrupt change in marine fossils at a certain stratigraphic point that later became known as the K/T boundary, the end of the Cretaceous period and beginning of the Tertiary (the "K" is from the German equivalent of "Cretaceous"). In the decades that followed there came to be interest in unusually large bones found in various sites around the world, and in 1841 the world learned that once upon a time the planet was roamed by terrible lizards -- dinosaurs. Eventually it became obvious that the dinosaurs disappeared at the end of the Cretaceous. Did they die because pesky little nocturnal mammals ate their eggs? Did they freeze in an ice age? Did their marshlands turn into mountain ranges? Did they get zapped by radiation from a nearby exploding star?

There were many outlandish theories. Constipation from eating flowering plants was one inventive explanation. So too was the idea, floated periodically, that an object from space smashed into the planet. In the early 1970s Nobel Prize winner Harold Urey pushed the impact hypothesis -- and hardly anyone paid attention to him. There was a problem with Urey's idea: It involved a catastrophe. Most scientists didn't like catastrophes. They were just a bit too . . . Old Testament.

Catastrophism had been popular until the early 1800s. Then came Sir Charles Lyell, a godlike figure who argued that the world was shaped by forces so gradual no human could perceive their workings. A canyon can be carved by the merest trickle of water. Mountains crept skyward millimeter by millimeter. Lyell was a uniformitarian, a believer that the laws of nature were constant in time and space, on Earth and in the heavens, now and forever. He ridiculed tales of "general catastrophes and a succession of deluges, of the alternation of periods of repose and disorder, of the refrigeration of the globe, of the sudden annihilation of whole races of animals and plants . . ." Such stories were not scientific. Charles Darwin then extended gradualism into the biological realm -- species, too, appeared and disappeared due to incremental genetic mutations and the imperceptibly subtle pressures of this vague phenomenon called "natural selection." Nature was the perfect subject for tweedy tenured professors using the same faded yellow lecture notes year after year.

Less than two decades ago the strange truth began to emerge: Both sides were right. Things happen slowly -- except when they happen suddenly.

During the 19th century there were many scientists who doubted that meteorites were, in fact, of extraterrestrial origin. Rocks fall from the sky? Preposterous! But slowly the evidence mounted. Geologists discovered that Meteor Crater in Arizona, once thought to be volcanic, had been formed by an impact only about 50,000 years ago. The legendary geologist Gene Shoemaker -- killed in a car accident last year -- convinced his colleagues that the craters on the moon were the scars of impacts. In the last two decades more craters, heavily eroded, have been found all over the world. The Chesapeake Bay basin turns out to be the remnant of an impact crater.

We now realize we live in a world that is not static but rather dynamic and tumultuous. The Old Testament had it right. Bad stuff happens. Things fall from the sky like thunderbolts from God. Small nervous children can be reassured -- they needn't worry about being eaten by a dinosaur, because sometimes most of the life on Earth is wiped out by a giant rock from space. Sweet dreams.

The link between the extinction of the dinosaurs and the Chicxulub crater did not happen instantly. In 1973 a geologist named Walter Alvarez began studying the K/T boundary near the medieval town of Gubbio, Italy. The Gubbio site is striking because the boundary is clearly delineated, with white rocks below and pink rocks above and, right at the boundary, a thin layer of clay. Alvarez called his father, Luis, a famous physicist, and wondered aloud how long it took for this layer of clay to be deposited.

Luis Alvarez realized that it would be possible to study the deposition rate of the K/T clay layer by measuring the abundance of the radioactive element iridium. He knew that iridium is scarce on Earth but comes to the surface from space in predictable quantities from the infall of small meteors and cosmic dust.

Walter Alvarez gathered samples of the K/T clay and passed it on to two researchers in a laboratory at Berkeley. Eight months later, the results came in. The iridium in the boundary was about 300 times more abundant than in adjacent rocks. The father-son team began to suspect that all this iridium came from a meteorite and had been spread around the globe in a catastrophic impact.

In 1979 Walter Alvarez presented his theory at a conference. He was scorned. What was he, some kind of old time fire-breathing catastrophist?

What no one knew was that, just a year earlier, Glen Penfield and Antonio Camargo, two geologists working for the Mexican oil company Pemex, had been making gravity maps and magnetic maps of the region around the Yucatan peninsula. They found a huge ringlike structure about 120 miles across. They thought it might be a remnant of a crater. Penfield and Camargo announced their finding at a conference in 1981. Carlos Byars, a reporter for the Houston Chronicle, knew of Alvarez's impact theory and quickly knocked out a story saying that perhaps this Yucatan structure was the hypothesized K/T crater. It was quite possibly one of the greatest journalistic scoops of all time -- and it was completely ignored. It just whooshed through the atmosphere unheard and unseen. Great work, no impact.

Years passed.

Geologists started noticing things. They noticed that there was evidence of a massive tidal wave far up the Brazos River in Texas. They noticed that sites around the Caribbean basin had heavy amounts of shocked quartz and glass fragments of the type produced by impacts. They were homing in on the crater of doom. The reporter Byars mentioned the suspected Yucatan crater to an Arizona graduate student named Alan Hildebrand, who in turn went to Penfield. He and Hildebrand reexamined the Chicxulub data. Everything seemed to fit, and they published the first scientific paper saying this was indeed the long-sought K/T crater. Pope and Ocampo followed shortly with their research showing that the sinkholes reveal the crater's edge.

Of course not everyone went along with the story. There were strenuous objections and alternative theories. There is much debate about when, precisely, the dinosaurs died out. No dinosaur fossils have ever been found right at the K/T boundary. Many dinosaur species went extinct long before the Chicxulub event. One theory is that vulcanism put the final kibosh on the dinosaurs. At about the time the last dinosaurs went extinct the planet was in a period of heavy volcanic eruption, particularly from a hot spot in India called the Deccan Traps. Geologist Charles Officer of Dartmouth argued that the volcanoes could have been the source of the iridium in the K/T boundary. The volcanoes could have caused global acid rain, destroying marine life at the base of the food chain. Officer has raised the possibility that Chicxulub may itself be the remnants of a volcano.

In Belize the competing theories provoked only huffing and snorting.

"Competely discredited," said one scientist.

But there is clearly much work to be done. Just because no one believes a theory doesn't mean it won't wind up a few years later as the unassailable orthodoxy.

We left Albion Island and drove south for half a day, toward the Maya Mountains. The vans took a shortcut from the Northern Highway to the Western Highway on a dusty road past the national penitentiary, which seemed completely empty, a circumstance possibly associated with the presence of escaped-prisoner posters on the walls of a nearby gas station.

Winding through the mountains we reached Pook's Hill, and a rain forest lodge run by two South African expatriates, Ray and Vicki Snaddon. They have cleared and mowed a series of terraces surrounding an ancient Maya ruin. Toucans flit among the towering palms. Iguanas climb all over one another in a cage near the edge of the forest. We hiked down a trail under the gloomy canopy. "Look at those vines -- just like Tarzan! Awesome!" said Tim Herman. We reached a river, and everyone jumped in, leaping from a boulder into a deep pool and dodging swarms of bats emerging from roots along the bank. After dark we ate beans and tortillas and then Tim Herman found a guitar and sang "Hotel California." A few people wandered off and studied the moths collected by an entomologist. They had been lured and hypnotized by a bright light behind a vertical net. They clung to the net and revealed their freaky Darwinian adaptations -- this one critter looked like an upside-down owl, complete with fake eyeballs on the back of his wings.

At considerable cost the expedition arranged for Internet access, and at the end of every day the explorers reported their discoveries and adventures on the Planetary Society's Web site, complete with uploaded photographs from the digital cameras. (One thinks of those technology-is-wonderful TV commercials where people in Banana Republic outfits get lost in the Amazon jungle and download a map from a satellite.) No one, however, had the slightest interest in downloading any information about the just-breaking Monica Lewinsky scandal. The explorers were grooving on geology and didn't seem to mind that we were now beyond the world of televisions, beyond the reach of CNN. They were operating on Geological Standard Time. In GST the gruntings and gropings of politicians are so ephemeral they barely register as part of reality.

By day we drove around and looked for road cuts, boulders, anything geologically intriguing. In a schoolyard everyone took samples of a mottled pink rock that had the distinct resemblance to the sandwich meat known as pork shoulder picnic. The Maya schoolkids used our magnifying glasses to study their hands and look at ants. A Maya man drove up and offered us a snake in a bottle. Bruce Hartel knew the species. "Very deadly. They drop on you out of the trees." He declined the offer.

At night we talked about cosmic questions of human destiny, extraterrestrial life, the nature of scientific knowledge, all sorts of big issues washed down with Belikan beers.

There is a theory, popular with those who imagine the universe teeming with technological civilizations, that the dinosaurs themselves would have evolved to possess the gift of intelligence. Dale Russell of the Canadian National Museum of Natural Sciences has argued that a dinosaur of the species stenonychosaurus showed signs of an enlarging brain. He built a model of a hypothetical descendant -- it looks like a bald, green man-osaur, strikingly similar to the aliens who supposedly visit us in UFOs.

This is the kind of thing one talks about in a jungle hut by lantern light.

Then the daytime comes and the search for real evidence becomes imperative, and the doubts creep in. Geology is a science of interpretation. Nothing is unambiguous. Rocks don't come with a label. You can't stick a probe into them and know their age and history. The theory of the Chicxulub impact isn't really controversial anymore, but there is much debate about the aftermath. Ocampo and Pope and their colleagues struggle to understand what exactly happened. Something unimaginably violent happened at this part of the world, and their job is to imagine it. Even the size of the crater isn't clear. About 120 miles across? Or 200 miles? Is the crater round or is it oblong? Did the impactor come straight down into Earth or hit at an angle?

Ocampo and Pope have convinced many influential geologists, including Walter Alvarez, that there truly is a thick ejecta blanket at the Albion quarry. More speculative, though, is their discovery of a possible type of ejecta farther south, near Pook's Hill. They have dubbed these rocks "Pook's pebbles." They have a strikingly polished quality, possibly from friction as they burned through the atmosphere. "There could be Pook's pebbles on the moon," Ocampo said. The most dazzling element of the pebbles are tiny craters in their surface filled with even tinier impactors. It looks as though these granules have penetrated the pebbles like bullets.

We looked one day for more pebbles and found them all over an exposed rock face in a garbage dump. But geologist Jan Smit said he wasn't convinced. Smit had provided the expedition with a world of expertise, having seen more K/T sites around the globe than any other human being. Smit thought the pebbles might simply have been weathered by water. It was just hard to tell. (Speak, rock!) Moreover, we couldn't date the geology in the garbage dump. Then we went to another road cut and found no pebbles. Everyone searched for fossils in vain. It was possible to sense a bit of frustration building.

"Do we really know anything?" Pope asked that night, covering his head with his hands. He was joking, a little.

Al Fischer, another team scientist, said, "Half of science is to wander around being confused."

Pope said that the story of the Pook's pebbles might not amount to a fully realized theory. "Maybe hypothesis is a better word for it."

The scientists lingered at the dinner table going over all the unanswered questions. The greatest mystery is how this Chicxulub impact caused so many extinctions. The dinosaurs and all those other creatures didn't simply get knocked over or burned up or drowned from the direct impact of the object. Wiping out an entire species is different from a mere mass killing. "An extinction is only true if you remove the last one," Jan Smit said. Pope asked, "Why couldn't you have pockets that survived?" To have such a vast and conclusive effect, the impact had to have triggered some kind of lingering global disaster.

The impact sent dust into the atmosphere that, combined with soot from continent-size wildfires, plunged the planet into darkness. How long this lasted is unknown, but there may have been a second problem: Sulfuric rock vaporized at the impact site would have filled the stratosphere and formed clouds, and they could have lingered for as long as a decade -- reflecting much of the sunlight back into space. Earth may have become a planet of vast ice sheets and glaciers. And then, when the clouds and dust were finally gone, it may have gotten hot. Really hot. This is because of another liberated material at the impact site, carbon dioxide. CO2 is a powerful greenhouse gas. Freeze them and then roast them: a recipe for extinction.

But now comes another quirky fact of the Chicxulub case. If that rock or comet had hit almost anywhere else on the planet, it wouldn't have caused so much damage, because it wouldn't have ejected so much CO2. Most of the world has a relatively thin layer of carbonate rock. But at the tip of the Yucatan is what is known as a carbonate platform, with carbon-rich limestone a couple of miles thick. According to Al Fischer, there weren't many places like that on the surface of Earth -- in addition to Belize, there were large carbonate platforms in the Bahamas, Seychelles and the northern coast of Australia. The carbonate platforms are found in clear waters along tropical shorelines with no nearby rivers to muddy things up. Those tourists who go to Belize to dive along the great coral reef know how perfectly clear the water is -- just right for the kind of creatures who create carbonate platforms, which, in turn, are capable of spewing massive amounts of carbon dioxide into the atmosphere in the extremely unlikely event that an asteroid or comet comes plowing into them.

Which is what happened! A contingency within a contingency. A bull's-eye within a bull's-eye.

What does this mean? It may mean that Chicxulub was a truly freakish event. But it may also suggest that a lot of other big rocks have hit Earth with less catastrophic consequences. Al Fischer said that scientists have identified 13 craters on the planet that are larger than 50 kilometers in diameter, but there are undoubtedly many more, particularly under the ocean where any trace would be hard to detect.

All this raises questions about what would actually happen in a "Deep Impact" situation. If we didn't divert the killer comet or get Stamper to blow it up (or did we just confuse movies?), the impact might not actually wipe out the human race or even permanently cripple civilization. Kevin Pope, who has drawn up all the horrifying diagrams of ejecta shooting into space and fire raining from the sky, predicts that humans would survive a Chicxulub-size impact. Things might get hairy for a while. One imagines tribal warfare, extremely barbaric conflict, lots of Mad Max scenarios.

But the essential feature of humanity is great adaptability. No other animal roams so much of the globe. We're no dinosaurs.

If there were any doubts that impacts are serious events in the solar system, just four years ago Comet Shoemaker-Levy put on a spectacular performance. Jupiter's gravity ripped the comet to pieces, which then peppered the giant planet like bullets from a machine gun. Astronomers saw, for the first time, what a catastrophic impact looks like. There were explosions on Jupiter that left brown, circular markings the size of Earth. The comet was just in time to give millennial fears a boost and help inspire Paramount to green-light "Deep Impact."

Early in the history of Earth there were so many impacts the planet was utterly uninhabitable. The all-time whopper of an impact, about 4.3 billion years ago, knocked so much of Earth's mantle into space that the debris collected and formed the moon. The period of great bombardment ended about 3.8 billion years ago when Earth finished sweeping the garbage and debris out of its orbital neighborhood. Evidence of life on Earth dates to about the same moment. The sweep-up job was not entirely complete, however. There are still some rocks with orbits that cross that of Earth. Comets can also cross Earth's path. There have been five major faunal extinctions in the last half-billion years, and it is possible that most or all of them were caused by impacts.

Programs to search for Near Earth Objects are either adequate or badly underfunded, depending on whom you ask. At the very least, it will take anywhere from 20 to 200 years to identify most of the asteroids that cross Earth's orbit. The next, slightly trickier question is what to do about the threat of bombardment. Edward Teller, father of the H-bomb, argues that we should build a system to nuke incoming asteroids. Skeptics say this is Teller's desperate attempt to find something useful to do with a terrible invention. Moreover, there are serious fears that a system designed to protect Earth from a catastrophic impact would be too easily used by a madman to imperil the planet instead. "The cure . . . may be worse than the disease," wrote the authors of a recent scientific paper. "We find that opportunities to misuse a deflection system are much more frequent than are occasions to use it for its intended purpose."

They point out, for example, that in the year 2070 an asteroid about a kilometer in diameter, named 1991 OA, will pass within a couple of million miles of Earth, and very close to the orbital trajectory of our planet. They argue it would need only a 60-megaton impulse -- large but not unimaginable -- to redirect it toward Earth.

One feels compelled to note that in all of human history there are no known cases of a person being killed by something falling from space. A meteorite bounced off someone's car fender once. Another one came through a lady's roof and crushed her radio and dinged her on the arm. An object about 200 feet across is thought to have exploded over the Tunguska region of Siberia in 1908, leveling hundreds of square miles of trees. People point out that the Tunguska object could have wiped out New York City, but the fact remains that it wiped out Tunguska -- a place so remote and uninhabited that its only chance of being mentioned in history books was to get hit with an asteroid.

It is extremely likely that you will not be killed by an asteroid or comet and that no one you know will be killed and that no other current inhabitants of Earth will be killed. Protection against asteroids and comets only becomes urgent if we decide we have a connection with life on Earth as it will exist far into the future. In fact, one way to look at the situation is to ask what kind of civilizations in the universe last a long time, and then to say: the ones that protect themselves from asteroids and comets. Astronomer Chris Chyba has written that impacts are part of a cosmic selection process. The universe favors technologically advanced creatures (particularly those who study space and build anti-asteroid systems) and selects against whales, chimpanzees, pigs, horses and other smart animals, not to mention dumb ol' dinosaurs. As Chyba puts it, "Dolphins can't protect the planet."

There is, however, a good reason why people shouldn't really worry about space bombs obliterating human civilization. It's that something else is more likely to get us first. There are terrors that exist not in Deep Time but in the here and now.

A biological weapon could take out half a million people in a matter of days. There remain thousands of nuclear weapons capable of being launched momentarily almost anywhere in the world (speaking of craters and a rain of fire). There are pathogens emerging from primeval forests. Antibiotics are putting selective pressure on mutant bacteria that are drug resistant. The latest worry is that the heating of the planet could melt the ice shelves of Antarctica and swamp coastal cities. Or will the altered atmosphere trigger a new ice age, complete with glaciers once again scouring across the northern United States and burying Manhattan under a mile of ice? (Yes, some of this is a good-news, bad-news situation.)

And then there's the really crazy stuff. The futurists talk about something they call the Singularity. The Singularity is the moment in the future when so many technologies have converged -- computers, miniaturization, biomedicine -- that they become autocatalytic, driving one another to yet greater sophistication, "hyper-accelerating," as Stewart Brand puts it. The Singularity is analogous to a black hole. No light or radiation or information escapes from the black hole of the future. We can make all the predictions we want, but they're worthless, because everything is changing so fast. There is an event horizon beyond which we can detect nothing. The future may be wonderful or it may be nightmarish, but either way there is the prospect of what the futurists call "severe discontinuity."

Stephen Hawking said at the White House recently that our destiny will be profoundly affected by genetic engineering -- we can alter our fundamental biology. As the night grows late that is where the conversation is headed, to a vision of our kind as semi-artificial, implanted with microchips, wired up to vast networks of information, a new species of creature, perhaps immortal.

Nature is not always static and gentle and gradual, and this is all the more true now that a new intelligent agent is upon the surface of the world, destroying habitats, altering the atmosphere, unleashing the forces of technology. There is no need for an extraterrestrial object to supply a deep impact upon Earth. We are the asteroid.

One night at Pook's Hill everyone gathered in the large thatched open-air hut for a lecture by Al Fischer. Fischer and his wife, Winnie, had driven all the way to Belize from their home in Los Angeles. They are of retirement age but hardly retiring. Fischer, in fact, had just seized upon a stunning idea. He planned to discuss it at a scientific conference in Rome but would first give us all a preview. Throughout his talk he wore shorts and a T-shirt and no shoes.

He drew a coiled line on the chalkboard, circling round and round like the shell of a nautilus. This was our planet, moving through time. Earth, he said, goes through a long, distinct cycle of heating and cooling. The cycle -- one circular trip around the chalkboard -- lasts 148 million years. We are nearing the coldest part of the cycle, so we're down near the bottom of the circle. What does this have to do with catastrophes? Fischer pointed out that the last time the planet was at a hot point, at the very top of the circle, was just about the time of the Chicxulub impact. In fact, more than half of the giant impacts known so far seem to have occurred when Earth was near the peak of the heating cycle. There are a few other impacts that are either at the bottom of the cycle (the coldest part) or halfway in between. The Chesapeake Bay impact, for example, occurred about 35 million years ago, halfway between the hottest and coldest phases.

These random, sudden, catastrophic, totally unforeseeable impacts are almost as regular as clockwork!

How could it be?

The galaxy, Fischer said. The solar system is orbiting the center of the Milky Way galaxy. The duration of an orbit is not precisely known, but Fischer said one estimate is about 300 million years -- or almost exactly twice the length of the hot/cold cycle! So there could be an additional pattern here. Earth goes around the galaxy, gets hotter and colder and hotter and colder, and at the hottest and coldest points finds itself bombarded with giant space objects.

He said he didn't know why the pattern exists. He said it might be related to the passage of the solar system through the two star-dense spiral arms of the galaxy. If the sun passes close to another star there could be a gravitational disturbance in the Oort Cloud, the community of comets that orbit the sun far beyond Pluto. Nudged from their home turf, comets could come flying in toward the inner solar system. Occasionally one would hit Earth.

By Fischer's calculation, we are about 8 million years from the next shower of comets. Eight million years until we bottom out on the hot/cold cycle. Eight million years to live?

"The whole thing has just developed over the last month," he said.

When he put down his chalk, everyone sat in awe. Tim the Californian . . . well, it just blew his mind. He announced, speaking for everyone, what an honor it was to be present on the night this incredible theory had been unveiled.

If Fischer was right, these impacts were not simply a planetary phenomenon. This whole thing was galactic. We were going into Ultra Deep Time here. That ghostly pale strip of light overhead on the darkest of nights -- that whirlpool of stars seen on edge -- is not some remote astronomical structure. It's our ecosystem!

It is the dream of science to find out how everything relates, to unify our understanding of Nature. On this night everyone wondered if we were getting closer. Maybe it was all somehow connected -- the galaxy, the climate, the craters, the bats flitting to and fro, the crawling iguanas, the freakish moths, the riotous trees in the dark moaning jungle. Air. Stars. A planet full of life.

In the morning we would go out and look at the world again.

Joel Achenbach is a reporter for the Style section of The Post. He is on leave writing a book about extraterrestrial life.