Great Ball of Fire

An artist's vision of how an asteroid's collision with Earth might have looked. The impact unleashed a chain of destructive forces.

Scientists call it "The Great Dying." nearly 250 million years ago, long before dinosaurs roamed, something terrible happened on Planet Earth. About 90% of all ocean species and 70% of those that lived on land were wiped out. It was the worst extinction ever. Huge coral reefs, forests of fernlike trees and ferocious reptiles were killed. A once common sea creature called the trilobite-an ancient relative of the horseshoe crab-disappeared forever.

What triggered this devastation? Last week scientists at the University of Washington announced they had cracked the puzzle. This "mother of all extinctions," as lead scientist Luann Becker calls it, happened when a giant asteroid or comet struck Earth. It was much like the one believed to have wiped out the dinosaurs 200 million years later.

Structures called buckyballs that contain space gases were found inside rocks in Japan, China and Hungary.

Evidence Trapped In Ancient Rock Becker's team found and studied tiny soccer-ball-shaped molecules, called buckyballs, trapped in 250 million-year-old rock. The molecules are made from gases that could have come only from a comet or an asteroid.

The scientists say the ancient collision triggered more than a thousand years of destruction. It caused volcanoes to erupt, heated up Earth's atmosphere and led to a sharp drop in the level of oxygen in the oceans.

Because Earth's surface has changed so much, no one knows just where the killer rock struck. But scientists can estimate its size: four to seven miles across!