Yet Sankar Chatterjee, a paleontologist at Texas Tech University in Lubbock, says the catalog overlooks several craters, including Shiva, a large, underwater crater off the coast of India. He says this crater measures 600 by 400 kilometers, and was made by an enormous meteorite measuring 40 kilometers across.

The Shiva crater is shaped like a teardrop, and Chatterjee thinks this is because the meteorite hit the Earth at a low angle.

"The K-T extinction was definitely a multiple-impact scenario," he says, pointing to two other craters not listed in the impact database: the Small Point structure off the coast of Maine, and the Silverpit crater in the North Sea.

These craters are not listed in the catalog because, despite the claims of their discoverers, they have not been independently confirmed to be the result of meteorite impacts. Doubt was cast on the impact origin of the Silverpit crater earlier this year, when it was reported in the journal Nature that Silverpit instead may be a sinkhole depression caused by salt withdrawal.

Christian Koeberl, a geochemist at the University of Vienna in Austria, says too many people are crying impact every time they see a round hole in the Earth's crust. Craters can result from many other natural processes, including volcanic eruptions.

Koeberl says that, ever since Chicxulub was confirmed as the likely cause of the K-T extinction, "now everybody gets on the impact bandwagon."

"A lot of people who have not the foggiest idea about how to really recognize an impact crater, and wouldn't be able to tell a shocked quartz grain from a tectonically deformed one if their life depended on it, call everything that is vaguely circular an impact crater," he says.

Other than being a round hole in the ground, an impact crater will have evidence of the sudden violent force that punched a hole in the Earth's crust.

For instance, there will be impact breccia, which is lighter, smashed-up rock that fills the crater after impact. Microscopic shards of "shocked quartz"-crystals that shattered in the shock waves of an impact - often will be present.

Minerals other than quartz, such as zircon, also may show signs of shock and exposure to high pressure. The heat of impact can often produce glass as well.

Geologists also look for an ejecta blanket radiating out from the crater. Above average amounts of iridium and other siderophile ("iron-loving") elements provide some of the strongest evidence for a meteorite impact, since those elements are rare on the surface of the Earth but can often be found in meteorites.

But not every impact crater will have all these attributes. Some meteorites don't contain iridium, for instance, so not every impact crater can be expected to be rich in that element.

Koeberl admits that identifying impact craters is neither easy nor straightforward, but he is adamant that Shiva is not an impact crater. Koeberl says not only is there no evidence of impact in the case of Shiva, there is no crater structure. He calls Shiva, "a figment of imagination."

"There's not even ambiguous evidence, or inconclusive evidence," says Koeberl. "There are a couple of people that keep pushing for some crater in the Indian Ocean, but this is inconsistent not only with the regional geology and geophysics, but also with anything we know about impact cratering."

Yet Chatterjee feels sure that Shiva is an impact crater. One indication of an impact origin, he says, is that the floor of the Shiva crater is missing most of the lithosphere - the brittle outer shell of the Earth that includes the crust (the continents and the ocean floor) and the uppermost part of the mantle.

Chatterjee says the large meteorite that created the Shiva crater could have easily shattered the lithosphere, and by doing so may have triggered plate tectonics. He says the rate of India's northward movement increased around 65 million years ago, and he suspects this was due to the Shiva impact.

Geologists who study plate tectonics agree that the Indian plate's northward movement did speed up, but say this acceleration probably occurred before the K-T extinction.

For instance, Jerome Dyment, a geologist with the Institut de Physique du Globe de Paris, says the plate sped up about 69 million years ago - moving from 8 to 18 centimeters per year. This faster rate was sustained for about 20 million years, and then slowed as India began to plow into the Eurasian continent.

At the time of the K-T extinction, India was an island located over the Reunion hotspot. Hotspots are fixed points where hot material from the mantle rises to the Earth's surface.

This underground welling flooded portions of India with a vast amount of lava. Today, these cooled lava fields are called the Deccan Traps.

The slow outpouring of Deccan lava probably began a few million years before the K-T extinction. Then about 65 million years ago, the trickle became a torrent.

Around the same time, says Steve Cande from the Scripps Institution of Oceanography in La Jolla, California, the India-Africa ridge jumped northwards to the edge of western India. This geologic "jump" caused a sliver of continent to split off, forming the Seychelles.

"These events are probably associated with the Deccan Traps," says Cande. "Now, I suppose you might say that all of these events were triggered by a meteorite impact, but I think most people believe that the Deccan Traps was the culmination of a mantle plume that was long in the making -- millions if not tens of millions of years."

While geologists haven't pinned down the exact connection between the ridge jump and the volcanic event that triggered the Deccan Traps, Dyment says that most believe both events may be the result of the head of the Reunion hotspot finally reaching the Earth's surface.

"Such large volcanic events and associated ridge jumps also have been observed in the Atlantic, " notes Dyment, pointing to similar activity in the north Atlantic near Iceland 54 million years ago and in the central Atlantic between North America and Africa 180 million years ago.

But Chatterjee believes the geologic activity in India is best explained by a massive meteorite impact. For further proof, he points to alkaline igneous rock spires that are encased in the Deccan Traps. These spires are rich in iridium, but the Deccan lava did not contain iridium. How else, he asks, could the spires have formed if not by a nearby meteorite impact?

In addition, Chatterjee says there is an underwater mountain as high as Mount Everest within the Shiva crater. He says this structure has been dated to be 65 million years old, and he thinks it could be the central peak that is often seen within large impact craters.

Finally, Chatterjee says the crater contains shocked quartz, a key sign of impact. And because the K-T clay boundary layer in India is one meter thick - the thickest in the world - Chatterjee thinks a meteorite impact must have been close by.

While all this evidence seems compelling, Chatterjee has so far failed to convince a majority of scientists that it adds up to proof of impact.

One problem, says Simon Kelley, a geologist at the Open University in England, is that there is not very much information about Shiva available in the peer-reviewed journals.

Chatterjee published a paper discussing Shiva eight years ago in the journal "Memoirs of the Queensland Museum," and Shiva is mentioned in a book about global tectonics that he edited. Most other information about the crater has appeared in conference abstracts or proceedings.

"The advance of science normally goes ahead by postulating hypotheses and then testing them with colleagues by publishing the work," says Kelley. "The lack of published work on Shiva means I can't really evaluate it against the normal criteria - so it has to be classed as a hypothesis to be tested."

Until Shiva can be studied more intensively, the crater will remain a tantalizing possibility rather than hard evidence of another K-T meteorite impact. Chatterjee says that oil companies and the Indian government control the site where Shiva is located, and access is extremely limited.

"It's very frustrating," says Chatterjee. "We are so close to solving the riddle, and yet so far because of the lack of critical drilling and geophysical data. If Shiva were indeed an impact crater, it would be the largest crater so far preserved on Earth."

The final part of this series will cover the debate over whether the K-T extinction was brought on by global warming. Read parts 1 and 2 of this series on the K-T extinction.

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