When Planet Earth Nearly Killed All Life

It's often known as "The Great Dying."

About 252 million years ago, all life nearly vanished. More than 90 percent of ocean species and 70 percent of land species died. Trees, plants, reptiles, fish, insects, microbes--all nearly wiped out.

The event marked the end of the Permian period and the beginning of the Triassic, the period that would give rise to the dinosaurs. But unlike the later Cretaceous extinction, the one that'd kill off the dinosaurs 186 million years later, this one wasn't caused by an asteroid impact, most scientists think. The ultimate trigger, in fact, has remained an ongoing mystery.

Scientists have unearthed many clues over the last decades, but they've yet to reach a final verdict. So far the evidence has pointed toward drastic changes on Earth and in the oceans. Oxygen levels were low in the oceans, making it difficult for many animals to survive. Carbon dioxide and methane levels were unusually high, contributing to major warming of the planet. Acid rain fell, and made sea water so acidic that all coral reefs disappeared. The inland turned into an dry desert, hot and arid, and devastated by major wild fires. The climate also varied between periods of sudden warming and cooling, making it impossible for many species to adjust.