Scientists said yesterday they have found evidence that a huge meteorite or comet plunged into the coastal waters of the Southern Hemisphere 251 million years ago, possibly triggering the most catastrophic mass extinction in Earth's history.

The researchers said that geological evidence suggests that an object about six miles in diameter crashed at the shoreline of what is now Australia's northwestern coast, creating climate changes and other natural catastrophes that wiped out 90 percent of marine species and 70 percent of land species.

This "extinction event" marks the boundary between the Permian and Triassic periods of geologic time. Another extinction that wiped out the dinosaurs and other species 65 million years ago is generally attributed to a similar-size meteor that punched an enormous crater in what is now Mexico's Yucatan Peninsula.

"What we found [is] the top of this very large feature buried under sediments," said Luann Becker, a University of California at Santa Barbara geochemist and leader of the research team reporting in the journal Science. "We have a date consistent with end-Permian [and] impact debris [minerals]. I think we have a pretty strong story."

Yesterday's report is the third published by Science documenting the Becker team's work. Nevertheless, the extraterrestrial impact theory remains controversial, because the extinction is also contemporaneous with a huge outpouring of volcanic "flood basalts" in Northern Siberia, perhaps also capable of triggering global climatic change.

"The evidence for impact has been growing over the last few years," said National Museum of Natural History paleontologist Douglas H. Erwin. "This isn't a slam-dunk yet for impact, but it sure makes it a stronger contender." However, he added, "scientists don't like coincidences, and we have the complication of Siberia."

The Permian-Triassic extinction is one of five such events known to have occurred through geological history, and is estimated to have caused twice as many species of plants and animals to become extinct as any of the others.

The trigger -- either an extraterrestrial impact or volcanism -- caused massive amounts of debris and dust to fly into the atmosphere, either creating a sudden greenhouse effect that led to radical global warming or obscuring the sun to such a degree that Earth was embraced by the prehistoric equivalent of "nuclear winter." A meteoric impact could also cause massive tidal waves and other disturbances.

Since 2001, Becker and her team, funded by NASA and the National Science Foundation, had reported finding evidence of impact debris in the southern reaches of what used to be the massive prehistoric continent Pangea. They described their latest work in a telephone news conference.

Team member Robert Poreda, an Earth scientist from the University of Rochester, said an Australian colleague tipped off the researchers about an underwater geological formation called the Bedout High that resembled Yucatan's Chicxulub Crater. The Permian deposits lay beneath almost two miles of ocean sediment.

After examining core samples taken from the formation by oil exploration firms, "we were absolutely flabbergasted," Poreda said. What had been described by the oil geologists as "volcanic rock" showed that bits of the crystalline mineral feldspar had been transformed into glass by the shock of the impact.

"This never occurs in volcanic debris," Poreda said. "This was absolutely convincing proof of an impact formation." Using dating techniques based on radioactive decay, the team found that a grain of material from the bottom of one of the Bedout oil exploration wells was 250.2 million years old -- close to the target date.

Finally, Poreda noted that last year, the team reported finding fragments of quartz and other minerals that had been similarly "shocked" by an extraterrestrial impact and spewed into the atmosphere to land 3,000 to 4,000 miles away in a pattern suggesting they came from the Bedout High.

In all, Poreda said, Bedout "happened at the right time, in the right place and had the right features" to match up with the extinction, "so, yes, this is a really good candidate."

Erwin acknowledged that the Becker team had provided additional evidence of an extraterrestrial impact, but "perhaps not yet compelling -- what's going to happen is that geologists and paleontologists will investigate this far more thoroughly and find more evidence to support or refute this interpretation."

He said the key task is to resolve the apparent conflict between the impact theory and the view that Siberian flood basalts caused the extinction. Scientists must make further efforts to accurately date both events to determine whether they are truly contemporaneous, he said.

If they are, he added, scientists must then determine whether the impact might somehow have caused the Siberian volcanism.