A white fireball two miles across thunders from the sky at 30,000 mph and crashes into the ocean off the Virginia coast. The impact vaporizes billions of tons of water, rips a hole in the sea floor six miles deep and fractures the bedrock far into the Earth.

The splash is 30 miles high. Debris is lofted over the horizon and rains down on an area of 3 million square miles, as distant as the Antarctic. Towering tsunamis surge toward the Blue Ridge Mountains.

Nearby life -- ferocious-looking sea creatures and dog-sized proto-horses along the tropical shoreline -- is blasted and then swept into the abyss by the boiling ocean. A calamity of unimaginable scale, it is probably the most stupendous geological event ever on the East Coast.

For more than a decade, geologists have believed that a gigantic object, an asteroid or a comet, struck the Earth north of Norfolk about 35 million years ago in a cataclysmic occurrence that left behind a 53-mile-wide, long-buried crater.

An international team of scientists, seeking clues to the origins of the planets, has assembled in a windblown bean field near the crater's center to try to determine, among other things, exactly what happened when the object struck.

Since early last month, the team has been working with a large drilling rig that uses diamond-tipped bits and brings up core samples to bore through eons of sediment toward the floor of the crater and the place where the impactor hit, believed to be about 7,000 feet below the surface.

As a farmer harvested his soybean crop just north of Cape Charles on Virginia's Eastern Shore and the wind off the Chesapeake Bay blew dust and grasshoppers across the drilling site, it was hard to imagine the scale of what geologists believe happened there.

"This is so big that we can't really picture it," said David S. Powars, a U.S. Geological Survey geologist, who said he first suspected the presence of an impact crater in the 1980s. "You could take the whole nuclear arsenal in its heyday: Russia, China, U.S. . . . That's a firecracker compared to what this explosion would be."

The men and women of the small but intense crater community who gather at the spot attempt to picture it every day. "I dream this all the time," Powars said. "People say, 'Did you sleep?' I say, 'I worked all night dreaming it.' I try, but I'll be honest: I can't imagine the event."

Their work is the culmination of a five-year project in which the USGS has drilled six holes probing the crater's landscape. This hole will be the program's deepest, and the last, officials say.

Since the formal announcement in 1995 of what is now called the Chesapeake Bay Impact Crater, studies have detailed its dimensions and outline, experts say. Last year scientists for the first time found rock that had been melted by the impact and fossils of microorganisms that had been smashed in the event.

There are scores of known impact sites around the world and millions more on planets and moons across the solar system. The one near Norfolk is Earth's seventh-largest site and the biggest in the United States.

On Earth, such impacts can dramatically alter the landscape in seconds, geologists say. And some scientists believe that understanding the moment of impact, "the soul . . . the spirit" of the collision, as one said, might be a key to understanding the formation of the solar system.

"If you think about how the Earth was formed," geologist Henning Dypvik of the University of Oslo said Wednesday at the drilling site. "The Earth was formed by a meteorite that came from here, an asteroid that came from there and [a] comet that came from here."

He moved his hands as if making a snowball. "This is the base process for the formation of the Earth and the universe," he said. "By studying [impacts], by understanding the mechanisms, then we can know much more about the Earth and the formation of the planetary system."

And then there is the question: What if such an object struck today? Even one a fraction of the size of the Chesapeake's would cause a disaster, said Powars, a Washington native and one of the people who discovered the crater. An impact by something a half-mile in size, and "the East Coast is in trouble," he said. "Lights out."

Impact science is fairly young, the geologists said. As recently as 20 years ago, the study of Earth impacts by "rocks . . . from heaven," as Dypvik put it, was considered crazy. The Earth's visible craters were thought to be remnants of volcanoes, he said.

Gradually, the scientific community realized that the Earth, like other planets, had been peppered over billions of years by renegade objects streaking through space. There are now more than 170 impact "structures" identified around the globe, more than 50 in North America.

The Earth's biggest, 186 miles across, is at Vredefort, South Africa.

The third-largest, the 100-mile-wide Chicxulub crater on Mexico's Yucatan Peninsula, is believed to be the result of an impact 65 million years ago that blew so much debris into the atmosphere that it darkened the Earth for months and led to the extinction of the dinosaurs.

Geologists don't believe that kind of thing happened after the Chesapeake impact. It "would have killed off the local population" for hundreds of miles up and down the coast, said Jean Self-Trail, a Geological Survey micropaleontologist. "But we don't really have any evidence that there was a massive die-off."

Small impacts happen almost all the time on Earth, said Jens Ormo, a Swedish crater expert working at Chesapeake site for the Spanish space agency. The big, so-called hypervelocity impacts are quite rare. He said one of the most recent occurred about 50,000 years ago and formed Arizona's Barringer crater.

The Chesapeake crater is the result of what geologists say was a marine impact. The object struck in several hundred feet of water far off the coastline, which was west of Richmond during the period of high global sea levels.

"It basically vaporized billions of tons of seawater," Powars said. "Billions of tons! And that's not exaggerating." There was a momentary hole in the water down to the sea bottom. "Then you had the water coming back in on this hot mixture of stuff, basically melted rock and sediments that fell back in . . . [creating] incredible steam explosions."

"It was a very volatile impact," Powars said. Another big marine impact crater in the Barents Sea off the northern coast of Norway is named Mjolnir, for the legendary hammer of the Scandinavian god Thor. There, geologists believe, the impact temporarily ignited sediments on the bottom of the ocean.

The Chesapeake crater, which geologists describe as shaped like an upside-down broad-brimmed hat, is centered near Cape Charles and extends north along the Delmarva Peninsula to about Wachapreague, Va. It goes west across the bay into Gloucester County, south to Norfolk and east into the ocean.

Powars said he first suspected the crater after routine geological drilling in the area found disorganized sediments underground. It was revealed after oil company explorations showed the outline of a crater. The impact object, probably an asteroid, disintegrated, the geologists said, but traces of it might be found if the drilling reaches deep enough.

The crater has been extremely well preserved because it is buried under land and sea sediment, the geologists said. But it is accessible via land and could become among the most-studied in the world.

"I often say how much I would love to see [an impact like] this happen," Powars said, "as long as I was on something that keeps me alive."

Two years ago, a pair of California scholars, Steven N. Ward and Erik Asphaug, published a paper about the potential impact of an asteroid that scientists think has a minute possibility of striking Earth in 2880.

The asteroid, called 1950 DA, is about a half-mile in diameter. The two men calculated that if it struck the sea at 40,000 mph, 370 miles off the East Coast, it would blow an 11-mile hole in the sea floor and within two hours would send 300-foot tsunamis crashing against the coast from Cape Cod, Mass., to Cape Hatteras, N.C.

The chances of impact are highly remote, and their paper was just "a focus of thought," the authors wrote. "Humanity lives with a calculus of infinite devastation times infinitesimal probability."

At the impact crater site on Virginia's Eastern Shore, a core sample from 421 feet reveals an embedded shell. The crater is the largest impact site in the United States.Chris Delahunty handles the long pipe used in the drilling. The rig's diamond-tipped bits dig through sediment.David S. Powars and Lucy E. Edwards, U.S. Geological Survey geologists, measure the core for sectioning.A core sample from the site, on Virginia's Eastern Shore.