Of course, any meteorite that slams into Earth has a 75 percent chance of hitting water. Since the ocean floor is fairly young, geologically speaking, and is continually recycled in deep ocean trenches, carrying the evidence of impact with it, scientists will never know about a lot of the hits Earth has taken. Richard Grieve, the pioneering Canadian impact expert, would love to see the detailed maps of the sea floor that the U.S. and Soviet navies made during the cold war, which undoubtedly show lots of intriguing holes. Despite talk in the 1990s of releasing the maps, he's not holding his breath. There's still "a fair level of paranoia" about declassifying the information, he says.
At least two suspected craters have turned up on the ocean floor. Two years ago, British oil geologists looking at seismic reflection data reported finding a 12-mile-wide multi-ringed impact structure in the North Sea. Much earlier-in the 1960s-a Navy oceanographic research vessel called the Eltanin found unusually high levels of the element iridium-which is abundant in meteorites-three miles beneath the Bellingshausen Sea, off Antarctica. Later expeditions to the site found meteorite fragments, and scientists now believe the Eltanin site was hit by an object up to a half-mile in diameter a mere 2.1 million years ago.
Dallas Abbott, a geoscientist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, claimed a couple of years ago to have found a whopping 80-mile-wide crater at the Eltanin site. Last year, though, a group of researchers in Germany and the United States disputed her claim in an unusually blunt paper delivered at a meteorite conference in Houston. They argued, based on their extensive research in the area, that Abbott "cannot possibly have found the Eltanin impact site."
Undaunted, Abbott made a truly astonishing claim at a geological meeting in Seattle last November. It wasn't so much the size (12 miles) of the crater-like ring she reported seeing in crude satellite-derived maps of the continental shelf off the coast of New Zealand. It was the age-only 500 years. Abbott offered a tantalizing grab bag of supporting evidence. She reported finding tektites in dredges taken near the crater-small beads of glass formed from cooled droplets of liquefied rock, which are also associated with impacts. She noted that other researchers have found tsunami deposits in Australia and New Zealand that some say are too high to have been dumped there by mere earthquake-generated waves. She pointed to curious Maori and aboriginal legends of a mysterious fire in the sky, images that, if based on an actual event, represent the only known account of a large impact.
So far, though, the critics remain unconvinced. An asteroid hit of that magnitude would have regional and perhaps global effects (a rough rule of thumb is that impactors are 1/20th the size of the crater, so this one would have been more than a half-mile wide). There's no clear evidence of such a devastating event circa A.D. 1500.
Other claims of young craters have been less controversial, but the scars are much smaller. The Wabar craters in Saudi Arabia, some of which are buried beneath the sand, were recently dated to 290 years ago, give or take a few decades. The largest is only about 380 feet in diameter. Two Arabic poems, which can be interpreted as referring to a meteor sighting from neighboring Yemen, may even give the exact date: September 1, 1704. And material collected from the proposed (but not yet added to The List) Sirente craters in central Italy dates from about A.D. 412. If those craters turn out to have been made by meteorites, the residents of late imperial Rome would have seen explosions more powerful than most humans have ever witnessed.
These signs of recent catastrophe-in fact, the entire catalog of 168 craters-do not help in the least in determining when Earth will be struck again. Planetologist Clark Chapman of the Southwest Research Institute in Boulder, Colorado, has estimated the frequency of large and small impacts based on other kinds of data, including ongoing telescope searches for "Earth-crossing" asteroids and military cameras that track fireballs in the sky and let scientists count smaller objects that burn up and never hit the ground. "Earth's geological record was important a couple of decades ago when we didn't have much data from a telescopic survey," says Chapman. "But now, instead of a handful of objects [that orbit in Earth's neighborhood], we know of literally thousands of them."
So even though finding new craters is fun, at present it's more of a scientific trophy hunt than a significant research program. Still, the pace of discovery is picking up as new tools, and new searchers, enter the game. Nearly a third of the craters known today were discovered in the last 15 years.
Every few months, a new claim crops up. Seneca County, Ohio. Nagano Prefecture, Japan. Grieve often has would-be crater discoverers bring him samples of rock they're certain contain shocked quartz. Identifying the deformation features can be tricky, but Grieve has seen them so many times it takes him only a few minutes with a microscope to give the visitor a yay or nay. He recalls the time a Chinese scientist brought him a sample. Grieve put it under the microscope. Nope. He handed it back to the researcher, who looked down at the rock, then back up at Grieve. "Chinese craters different," he said.
Kevin Evans, who expects Weaubleau-Osceola to be added to The List someday, doesn't claim to be an impact expert. He says, "I'm just kind of an interloper," one who normally works on plain old sediment geology. Conducting research in Antarctica had been one of the thrills of Evans' career, but of the Weaubleau-Osceola discovery he says, "This is the most exciting project I've ever been on."