Whenever I can escape the gloomy winter months up north, I enjoy walking the sandy beaches of Floridas popular Sanibel (San Ybel) Island along the Gulf of Mexico. I especially enjoy beachcombing, picking up the many shells, corals, and sponges that wash ashore on this subtropical island. At night the stars above the gulf are ablaze overhead and you can even spot the top of the constellation of the Crux, or Southern Cross, as it begins to emerge at the low latitude of Sanibel. But it came as a surprise to learn recently that I was walking along the eastern rim of an immense, prehistoric impact crater. Dr. Michael Stanton, a retired Canadian geologist, recently proposedwhile pointing to considerable physical evidencethat the Gulf of Mexico was formed 250 million years ago by the impact of a large asteroid or comet. As a huge cosmic-impact feature, Stanton said, the Gulf of Mexico is the terrestrial equivalent of the maria (seas) on the Moon. In fact, the gulf is nearly identical in shape to the Moons Mare Orientale although this lunar mare is smaller than our gulf in diameter: 900 km (560 miles) versus 1,500 km (810 miles). If Stantons theory is correct, and it appears to be a rock-solid case, then the 250 million year-old gulf impact would have been the likely culprit behind the Permian-Triassic mass extinctions. Over 96 percent of all marine animal species and 70 percent of terrestrial vertebrate species became extinct at the close of the Permian Period. This impact event also must have contributed to the mysterious Permian glaciation in the southern part of Pangea, the prehistoric supercontinent. When you look at a map of the Gulf of Mexico, its not hard to see what Stanton sees: The nearly circular feature of the gulf even looks like a vast, albeit distorted, crater rim. Under the gulf is where Stanton found even more evidence pointing to a cosmic catastrophe on an apocalyptic scalethe Permian impacts force was so titanic that it resulted in an uplifted Moho, the boundary between the Earth's crust and the mantle. The impact also metamorphosed (melted) all the ancient Paleozoic rocks underlying the gulf and created a vast network of faults and grabens, depressed blocks of land that are bordered by parallel faults. After the big impact that formed todays Gulf of Mexico, the molten basin acted as a vast evaporating pan for the deposition of the famous Louann Salt, a huge deposit of sea salt under the gulf nearly 5,000 feet thick. And finally, the gulf impact was so massive that it unleashed the continental separation of Pangea at ground zero. Stanton believes the old, eroded Ouachita Mountains in west central Arkansas and southeastern Oklahoma were upthrust as a result of the gulf impact. Next time you look at a map or a satellite image of the Gulf of Mexico, think impact crater, Stanton said. It works for me.