WASHINGTON - It was the greatest mass murder of all time -- poison everywhere! billions slain! -- 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, 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 close to pinning down what caused the extinction of most plants and animals on Earth about 251 million years ago.

The perpetrator wasn't an asteroid or comet, like the impact thought to have wiped out the dinosaurs 65 million years ago.

Instead, it was a cascade of events that began with a monstrous outpouring of 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 multi-million year geologic period, the Permian, and the beginning of another, the Triassic.

A cautionary tale?

Besides being 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.

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 carbon dioxide "threshold" that could set off a similar environmental upheaval.

How it happened

Exactly what caused the ancient mass extinction is still unclear, but here's how many researchers think it unfolded:

Over a period of about a million years, lava from deep in the Earth's interior oozed up through giant cracks in Siberia's crust. The molten mass "froze" into step-like slabs of volcanic rocks known as the Siberian Traps.

The lava's exit 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 ocean currents that keep the waters circulating. The oceans lost most of their oxygen. Marine plants and animals suffocated.

What happened to snuff out life on land is still debated. Some researchers think 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 gas emerged from the sea, it choked half of all land creatures.

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