WASHINGTON – It was the greatest mass killing of all time, but the killer or killers have never been positively identified.

An estimated 95 percent of all marine species and up to 85 percent of land creatures perished amid seas and air full of poisons, 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 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.

The slaughter is formally known as the Permian-Triassic Mass Extinction because it marked the end of a multimillion-year geological period, the Permian, and the beginning of another, the Triassic.

To further unravel the mystery, the National Science Foundation has launched an international project to study the Siberian lava, led by Linda Elkins-Tanton, a geologist at the Massachusetts Institute of Technology. "We have 28 scientists from seven countries and five years' worth of funding," Elkins-Tanton said. "I'm very excited about it!"

A puzzling detective story, the P-T extinction is also a cautionary tale for our time.

"The end-Permian catastrophe is an extreme version of the consequences of global warming," said Lee Kump, a geoscientist at Pennsylvania State University. "It reminds us that there are unexpected consequences of CO 2 buildup, and these can be quite dire."

The lessons of the P-T massacre are "directly applicable to the present," said John Isbell, a geoscientist at the University of Wisconsin in Milwaukee. He said the world today is in danger of exceeding a CO 2 "threshold" that could set off an upheaval as great as the one 251 million years ago.

Isbell said CO 2 levels in the atmosphere at the time of the P-T catastrophe reached 1,000 to 1,500 parts per million, far higher than today's level of 385 ppm. CO 2 levels are now rising by 2 ppm a year, and that's expected to accelerate to 3 ppm a year. If carbon emissions aren't reduced, some researchers fear that by the end of the next century, the CO 2 level could approach what it was during the P-T period.

Exactly what caused the ancient mass extinction is still unclear, but here's how many researchers think it may have unfolded: Over a period of about a million years, an enormous amount of lava from deep inside the Earth oozed up through giant cracks in Siberia's crust. The molten mass "froze" into step-like slabs of flood basalts, volcanic rocks known as the Siberian Traps.

Enough lava gushed out to cover an area almost as large as the continental United States.

The lava from the Siberian Traps sent huge quantities of carbon dioxide and methane into the atmosphere. These greenhouse gases caused an epic spell of global warming. Toxic acid rain drizzled from the sky, and the ozone shield in the atmosphere thinned, letting deadly ultraviolet radiation pass through.

As is happening now, the Earth warmed more near the poles than it did at the equator. The smaller temperature difference slowed the great currents that keep the oceans' waters circulating. The oceans stagnated and lost most of their oxygen. Marine plants and animals suffocated.

What happened to snuff out life on land is still debated. Some researchers believe that bacteria in the ocean, living on sulfur instead of oxygen, churned out vast quantities of hydrogen sulfide, a lethal gas with a rotten-egg smell. As the hydrogen sulfide gas emerged from the sea, it choked half of all land creatures.

Call Robert S. Boyd, McClatchy Washington Bureau, (202) 383-6007.