On to science! Everyone knows that a comet or asteroid impact may have killed the dinosaurs. (Well, almost everyone. If you believe that the world is six thousand years old and was created in seven days, the rest of this column will just needlessly irritate you.)

Most people are unaware, however, that the greatest mass dying we know of occurred tens of millions of years before the end of the dinosaurs, a quarter of a billion years ago at the beginning of the age of the dinosaurs.

For a long time, scientists have been trying to figure out what caused that mass extinction, the one that divides the Permian from the Triassic. Since the discovery of the Chicxulub crater in Yucatan, Mexico, a leading contender has been the idea that the Permian-Triassic extinction was caused by an unknown earlier impact. Now, a crater has been discovered which is deep under the ice of Antarctica. It may have formed 250 million years ago, which would put it at the right age for the Permian-Triassic extinction.

The crater is three hundred miles in diameter. It was possibly caused by a meteor thirty miles wide. If another like it struck today, it would kill pretty much every human on Earth.

If the crater is from a quarter of a billion years ago, it's in the right time and place to have killed off most of the species then living on Earth, and to have initiated the breakup of the supercontinent Gondwanaland.

Information on the Antarctic crater is here, here, here, and here.