A massive dent hidden under the seabed off the north west coast of Australia could be evidence of a meteor impact responsible for the biggest mass extinction in Earth's history, scientists say.

A team led by Dr Luann Becker, University of California, Santa Barbara report their findings in this week's issue of Science Express, the online journal of Science Magazine.

The world's largest mass extinction, often called the Great Dying, took place 250 million years ago when 90% of marine life and 80% of life on land were wiped out. While the extinction of the dinosaurs some time later has been at least partly blamed on a meteor impact, there has been little evidence to date that a similar event triggered the Great Dying at the end of the Permian.

Now, all that may have changed.

Becker and team think they may have found a late Permian impact crater at "Bedout High" [pronounced Bedoo], a 200 kilometre-wide dent under the seabed, 250 km due north of Port Hedland in Western Australia.

Seismic studies, carried out by team member Robert Iasky of the Geological Survey of Western Australia in Perth, shows the Bedout structure has a central uplift of rocks surrounded by a basin.

Iasky told ABC Science Online that some aspects of the structure are "very similar in character" to the Chicxulub crater in Mexico which is believed to be the crater left by the meteor which wiped out the dinosaurs.

The research team also argues the Bedout structure contains rocks which are characteristic of debris from extraterrestrial impact sites and are of about the right age. Earlier work by lead author, Becker had shown similar "fallout" in China, Antarctica and the Sydney basin, all pointing to an extraterrestrial impact somewhere in western Australia, Iasky said.

Analysing cores originally collected by oil companies during exploration in the 1970s and 1980s, Becker and team report finding a type of black glass called maskelynite, which is known only to be formed under the extreme temperatures and pressures of an extraterrestrial impact.

However some scientists have already debated the team's interpretation that they have found maskelynite, as opposed to a similar-looking glass produced by volcanoes.

"I've never seen maskelynite look like this," impact geologist Dr Richard Grieve of the Canadian Geological Survey told Science Magazine's news service.

"That's a sore point," Iasky told ABC Science Online, acknowledging the findings were debatable. "The paper will raise a lot of questions."

He said the definitive evidence would have been "shocked quartz" - a particular type of disfigured quatz which is unmistakably formed from an extraterrestrial impact and has been found at the famous Chicxulub crater.

"If they had found shocked quartz at Bedout, it would not be a possible impact crater, it would definitely be an impact crater."

Despite scouring the cores for this definite signature, the team did not find it. The main reason, said Iasky, is that shocked quartz tends to appear beneath the crater surface itself rather than in the debris on top of it. But the cores taken by the petroleum company decades ago did not go down far enough, and today it would cost A$5 million to drill further.

Alternatively, at the end of the Permian there was a lot volcanic activity and movement of land masses which could in itself have produced the "Bedout High" - albeit in a strikingly similar shape to known impact craters.

"The structure looks like it could quite well be an impact crater - although it could be a volcanic uplift," said Iasky. "Although I'm a co-author, I'm sitting on the fence."

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