HIDDEN deep below the flat scrub land along Mexico's Yucatan peninsula and the tranquil blue sea offshore is the giant imprint of a cosmic catastrophe.
Millions of years before humans existed, a huge meteorite measuring about six miles across and weighing perhaps billions of tons crashed into the Earth. It left one of the biggest craters ever made in the planet and probably caused a global environmental disaster, shrouding the earth in a dense cloud of dust that blocked out sunlight and sent temperatures plummeting.
Invisible to the millions of tourists crossing the Yucatan each year to visit its famous Maya ruins, the Chicxulub crater (pronounced CHICH-oo-loop) is now being studied by a British-led team of scientists who believe it might explain why the dinosaurs suddenly became extinct.
The scientists, led by Mike Warner of London University's Imperial College, are excited because the age of the crater - about 65 million years - coincides with a period in which hundreds of species, including the dinosaurs, suddenly died out.
"It's extremely likely that the meteorite impact was linked to the many plants and animals which became extinct at the same time as the dinosaurs," Mr. Warner said in a telephone interview. "But making a causal link to the dinosaurs is difficult. You can say it's quite likely, but it's very hard to make a firm link."
Warner and his team say their study of the Chicxulub crater, named after the small Yucatan fishing port near to its center, might give them hard evidence. One of the most important issues is the crater's size. The bigger it is, the more likely it is that the impact was to blame for the mass extinctions.
The top estimates put the crater at 180 miles across and the lowest at 115 miles, so the team is placing measuring equipment at 20 sites along lines drawn across the basin of the crater to determine its true dimensions. The highly sensitive equipment picks up shock waves from the frequent small earthquakes that ripple across Mexico several times a month. Depending on how the waves are transmitted through the rock deep below the earth, scientists can draw conclusions about the crater's structure and size.
Warner estimates that the force of the meteorite hitting the Earth "might have been equivalent to an earthquake measuring 12 or 13 on the Richter scale." Such a massive quake has never been recorded. It would be about 10,000 times stronger than the one that leveled San Francisco in 1906 and equal to the explosive force of hundreds of atomic bombs.
"The effects of this meteorite have been compared to that of a nuclear winter," says Graeme Mackenzie, a Leicester (England) University researcher on the project team in Chicxulub. "There would have been a lot of dust and sulfur thrown up into the atmosphere, [and] strong acid rain."
Paul Denton, a Leicester University scientist, says 60 percent of all recorded species on the earth disappeared around the time the meteorite struck. On land, nothing larger than a dog survived.
The crater was discovered only recently. Oil geologists from Mexico first found it while hunting for crude oil in the 1970s. They mentioned the find in one line of a brief scientific paper written in Spanish in 1980 that went unnoticed for another decade until scientists realized its importance.
Earthlings need not fear another huge meteorite hitting the planet soon. "One that size," Mackenzie says, "only hits about every 100 million years."
[In a related story, the Associated Press reports that scientists believe they have found a chain of impact craters in the central African nation of Chad. It may have formed when a large, fragmented asteroid or comet hit the Earth about 360 million years ago. Scientists detected evidence of the craters in radar images taken from the space shuttle Endeavour in 1994. Adriana Ocampo, a geologist with NASA's Jet Propulsion Laboratory in Pasadena, Calif., presented her findings at a Houston conference March 20.
[Ms. Ocampo believes the comet or asteroid fragments that hit Chad were about one-third to two-thirds of a mile in diameter and the resulting craters all measured about 7.5 to 10.5 miles wide. The impacts may have contributed to a period of mass biological extinction seen 360 million years ago.]