Ralph von Frese argues that a structure in the crust under Antarctica is a 500km wide impact crater and speculates that is was the source of the Permian-Triassic Extinction event which killed off 90% of all marine species.
Confirmation of this theory would require at least two things: the presence of shocked minerals near the impact crater, and dating of the crater to near the boundary (245 million years ago). It has been assumed by many that if there was an extraterrestial impact that caused the extinction it had hit in an ocean basin, and the evidence on the sea floor had long been subducted (the maximum age of oceanic crust in the Pacific is Jurassic in age).
Some images are here.
EDIT: There is some counter-arguments at Nature. Link via The Esoteric Science Resource Center.
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