Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the past 10,000 years. But the self-described "band of misfits" that make up the two-year-old Holocene Impact Working Group say astronomers simply have not known how or where to look for evidence.

Scientists in the working group say the evidence for such impacts during the past 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to 1 million years, as astronomers now calculate, catastrophic impacts could happen every few thousand years.

At the southern end of Madagascar lie four enormous wedge-shaped sediment deposits, called chevrons, that are composed of material from the ocean floor. Each covers more than 100 square kilometers with sediment hundreds of meters deep.

On close inspection, the chevron deposits contain deep-ocean microfossils that are fused with a medley of metals typically formed by cosmic impacts. And all of them point in the same direction - toward the middle of the Indian Ocean where a newly discovered crater, 29 kilometers, or 18 miles, in diameter, lies 3,800 meters, or 12,500 feet, below the surface.

The explanation is obvious to some scientists. A large asteroid or comet, the kind that could kill a quarter of the world's population, smashed into the Indian Ocean 4,800 years ago, producing a tsunami at least 183 meters thigh, about 13 times as big as the one that inundated Indonesia nearly two years ago. The wave carried the huge deposits of sediment to land.

Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the past 10,000 years. But the self-described "band of misfits" that make up the two-year-old Holocene Impact Working Group say astronomers simply have not known how or where to look for evidence.

Scientists in the working group say the evidence for such impacts during the past 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to 1 million years, as astronomers now calculate, catastrophic impacts could happen every few thousand years.

The researchers, who formed the working group after finding one another through an international conference, are based in the United States, Australia, Russia, France and Ireland. They are established experts in geology, geophysics, geomorphology, tsunamis, tree rings, soil science and archaeology, including the structural analysis of myth. Their efforts are just getting under way, but they will present some of their work at the American Geophysical Union meeting in December in San Francisco.

This year the group started using Google Earth, a free source of satellite images, to search the globe for chevrons, which they interpret as evidence of past giant tsunamis. Scores of such sites have turned up in Australia, Africa, Europe and the United States.

When the chevrons all point in the same direction to open water, Dallas Abbott, an adjunct research scientist at Lamont-Doherty Earth Observatory in Palisades, New York, uses a different satellite technology to look for oceanic craters. With increasing frequency, she finds them, including an especially large one dating back 4,800 years.

So far, astronomers are skeptical but willing to look at the evidence, said David Morrison, a leading authority on asteroids and comets at the NASA Ames Research Center in California.

Surveys show that as many as 185 large asteroids or comets hit the Earth in the far-distant past, although most of the craters are on land. No one has spent much time looking for craters in the deep ocean, Morrison said, assuming that young ones do not exist and that old ones would be filled with sediment.

Astronomers monitor every small space object with an orbit close to the Earth. "We know what's out there, when they return, how close they come," Morrison said. Given their observations, "there is no reason to think we have had major hits in the last 10,000 years," he continued, adding, "But if Dallas is right and they find 10 such events, we'll have a real contradiction on our hands."

Peter Bobrowski, a researcher of natural hazards at the Geological Survey of Canada, said that "chevrons are fantastic features" but do not prove that megatsunamis are real. There are other interpretations for how chevrons are formed, Bobrowski said, and it is up to the working group to prove its claims.

Ted Bryant, a geomorphologist at the University of Wollongong in Australia, was the first person to recognize the palm prints of megatsunamis. Large tsunamis of nine meters or more are caused by volcanoes, earthquakes and submarine landslides, he said, and their deposits have different features.

Deposits from megatsunamis contain unusual rocks with marine oyster shells, which cannot be explained by wind erosion, storm waves, volcanoes or other natural processes, Bryant said.

"We're not talking about any tsunami you're ever seen," Bryant said. "Aceh was a dimple. No tsunami in the modern world could have made these features. End-of-the-world movies do not capture the size of these waves."

For example, Bryant identified two chevrons found more than six kilometers inland near Carpentaria in north- central Australia. Both point north. When Abbott visited a year ago, he asked her to find the craters.

To locate craters, Abbott uses sea-surface altimetry data. Satellites scan the ocean surface and log the exact height of it. Underwater mountain ranges, trenches and holes in the ground disturb the Earth's gravitational field, causing sea-surface heights to vary by milimeters. Within 24 hours of searching the shallow water north of the two chevrons, Abbott found two craters.

"We think these two craters are 1,200 years old," Abbott said. The chevrons are well-preserved and date to about the same time.

Abbott and her colleagues have located chevrons in the Caribbean, Scotland, Vietnam and North Korea, and several in the North Sea.

But Madagascar provides the smoking gun for geologically recent impacts. In August, Abbott, Bryant and Slava Gusiakov, from the Novosibirsk Tsunami Laboratory in Russia, visited the four huge chevrons to scoop up samples.

Last month, Dee Breger, director of microscopy at Drexel University in Philadelphia, looked at the samples under a scanning electron microscope and found benthic foraminifera, tiny fossils from the ocean floor, sprinkled throughout. Her close-ups revealed splashes of iron, nickel and chrome fused to the fossils.

When a chondritic meteor, the most common kind, vaporizes upon impact in the ocean, those three metals are formed in the same relative proportions as seen in the microfossils, Abbott said.

Breger said the microfossils appear to have melded with the condensing metals as both were lofted up out of the sea and carried long distances.

About 1400 kilometers southeast from the Madagascar chevrons, in deep ocean, is Burckle Crater, which Abbott discovered last year. Burckle Crater has not been dated, but Abbott estimates that it is 4,500 to 5,000 years old.

It would be a great help to the cause if the National Science Foundation sent a ship equipped with modern acoustic equipment to take a closer look at Burckle, said William Ryan, a marine geologist at the Lamont-Doherty observatory. "If it had clear impact features, the nonbelievers would believe," he said.

But they might have more trouble believing one of the scientists, Bruce Masse, an environmental archaeologist at the Los Alamos National Laboratory in New Mexico. He thinks he can say precisely when the comet fell: on the morning of May 10, 2807 B.C.

Masse analyzed 175 flood myths from around the world and tried to relate them to known and accurately dated natural events like solar eclipses and volcanic eruptions. Among other evidence, he said, 14 flood myths specifically mention a full solar eclipse, which could have been the one that occurred in May 2807 B.C.

Half the myths talk of a torrential downpour, Masse said. A third tells of a tsunami. Worldwide, they describe hurricane-force winds and darkness during the storm. All of these could come from a megatsunami.

Of course, extraordinary claims require extraordinary proof, Masse said, "and we're not there yet."