A mile beneath the East Antarctic ice sheet. Theory is that a 30 mile meteor smacked the earth 250 million years ago, splitting Gondwana's plate and causing the Permian-Triassic extinction.
The surprising part is that 30% of land vertebrates survived! Land critters are hard to kill, 90% of sea critters died.
The crater sounds pretty real, its relation to the extinction is said to be more controversial.