What appears to be a 480km-wide (300 miles) crater has been detected under the East Antarctic Ice Sheet.
Combined gravity and radar data reveal a crater formation deep under the ice (Image: OSU)
The scientists behind the discovery say it could have been made by a massive meteorite strike 250 million years ago.
The feature at Wilkes Land was found by Nasa satellites that are mapping subtle differences in the Earth's gravity.
"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs," said Prof Ralph von Frese, from Ohio State University, in the US.
If the crater really was formed at the time von Frese and colleagues believe, it will raise interest as a possible cause of the "great dying" - the biggest of all the Earth's mass extinctions when 95% of all marine life and 70% of all land species disappeared.
Some scientists have long suspected that the extinction at the boundary of the Permian and Triassic (PT) Periods could have occurred quite abruptly - the result of environmental changes brought on by the impact of a giant space rock.
It is a similar argument to the one put forward to explain the demise of the dinosaurs at the much later date of 65 million years ago.
The greatest of all Earth's mass extinctions occurred about 250 million years ago
About 95% of marine species and three-quarters of all families on the Pangean (above) landmass perished
Rocks from the end of the Permian period can be seen today in places such as China, Italy and Pakistan
Chief suspects include sea-level fluctuations, volcanic activity, space impacts and melting methane-ice in sea sediments
A geological structure, known as the Bedout High, in the seabed off what is now Australia, has also been suggested as the possible crater remains from the PT impactor.
But the impact explanation for the great dying is an argument that has struggled to find favour.
The prevailing theory is that several factors - including supervolcanism and extensive climate warming - combined over thousands of years to strangle the planet's biodiversity.
Earth may well have been hit by extraterrestrial objects, but it is unlikely there was some killer punch from space, these other researchers contend.
The Ohio-led team used gravity fluctuations measured by the US space agency's Grace satellites to peer beneath Antarctica's icy surface. Team members were drawn from the US, Russia and Korea.
The crater information was first presented at the recent American Geophysical Union Joint Assembly in Baltimore.