Geologists detected the crater through smashed mineral grains and magnetic and gravity measurements of the region around the town of Woodleigh, near Shark Bay on Australia's west coast.

Although no precise age for the crater -- the fourth largest in the world -- has been determined yet, it appears to be 250-360 million years old, making it a possible source for the massive Permian-Triassic extinction event that wiped out almost all life on the planet 250 million years ago.

The Permian-Triassic die-off destroyed 96 percent of all sea life and nearly as much on land. It dwarfs the 65-million-year-old Cretaceous-Tertiary extinctions that eradicated 75 percent of species, including the dinosaurs.

"The global environmental effects would have been catastrophic," said Geological Survey of Western Australia geoscientist Robert Iasky, who, with colleague Arthur Mory, hunted down the crater.

The researchers reported their find in the April 15 issue of Earth and Planetary Science Letters.

The Woodleigh impact crater ranks fourth largest on Earth, after the 200-mile-wide Vredefort crater in South Africa, the 165-mile wide Sudbury crater in Canada and the famous 120-mile-wide alleged dino-killing Chicxulub impact crater in the Gulf of Mexico.

The timing of the Woodleigh crater, rather than its size, makes it a candidate for the cause of the massive Permian-Triassic extinction event. The global damage caused by a meteor or comet impact varies greatly according to size, speed, the angle it strikes the Earth and whether it hits on land or sea.

Although petroleum-prospecting geologists first happened upon evidence of the crater while drilling in the red-sand region in the 1970s, they didn't realize what they had found. It wasn't until 1997, when a gravity survey of the area revealed a round structure with a telltale rebound dome in the center, that suspicions of a crater were roused.

In 1999, geologists returned to the hole drilled over the crater's central mound and drilled deeper -- collecting "shocked" grains of quartz that are strong evidence of an impact.

"We identified this as being a product of extreme pressures during severe shock caused by an extraterrestrial impact," said Iasky. "In other words, a meteor impact."

"There doesn't seem to be any doubt about it," said earth scientist and impact specialist Michael Rampino of New York University.

It's also fairly certain such a huge impact caused extinctions, he said. But whether it caused the great Permian-Triassic extinctions remains to be seen. "It's tantalizing but not proven," Rampino said.