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This image is an artist's impression of a Lower Permian swamp in Texas.The terrible event had been lost in the amnesia of time for eons.It was only recently that paleontologists, like hikers stumbling upon an unmarked grave in the woods, noticed a startling pattern in the fossil record: Below a certain point in the accumulated layers of earth, the rock shows signs of an ancient world teeming with life.

Listen to this story via streaming audio, a downloadable file, or get help. Some perpetrator -- or perpetrators -- committed murder on a scale unequaled in the history of the world.

They left few clues to their identity, and they buried all the evidence under layers and layers of earth.

The case has gone unsolved for years -- 250 million years, that is.

But now the pieces are starting to come together, thanks to a team of NASA-funded sleuths who have found the "fingerprints" of the villain, or at least of one of the accomplices Above: Life was flourishing on the Earth about 250 million years ago, then during a brief window of geologic time nearly all of it was wiped out.

Somehow, most of the life on Earth perished in a brief moment of geologic time roughly 250 million years ago.

Scientists call it the Permian-Triassic extinction or "the Great Dying" -- not to be confused with the better-known Cretaceous-Tertiary extinction that signaled the end of the dinosaurs 65 million years ago.Whatever happened during the Permian-Triassic period was much worse: No class of life was spared from the devastation.Trees, plants, lizards, proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped out.Roughly 9 in 10 marine species and 7 in 10 land species vanished. Scientists have suggested many possible causes for the Great Dying: severe volcanism, a nearby supernova, environmental changes wrought by the formation of a super-continent, the devastating impact of a large asteroid -- or some combination of these.Proving which theory is correct has been difficult.The trail has grown cold over the last quarter billion years; much of the evidence has been destroyed.