An estimated 95 percent of all marine species and up to 85 percent of land creatures perished, according to Peter Ward, a paleobiologist at the University of Washington in Seattle.

Scientists call it "The Great Dying." Life took millions of years to recover.

Scientific sleuths, however, now think they're making progress toward pinning down what caused the extinction of most plants and animals on Earth some 251 million years ago.

The perpetrator wasn't an asteroid or comet, like the impact that wiped out the dinosaurs 65 million years ago and inspired movies such as Deep Impact and Armageddon.

Instead, it was a cascade of events that began with a monstrous outpouring of hot, reeking lava in Siberia. Repeated floods of lava released massive amounts of carbon dioxide, which produced a runaway greenhouse effect, oxygen-starved oceans and a poisoned atmosphere.