Some 250 million years ago, the earth suffered the greatest biological crisis in its history. Around 95% of all living species died out–a global catastrophe far greater than the dinosaurs demise 65 million years ago. How this happened remains a mystery. But there are many competing theories. Some blame huge volcanic eruptions that covered an area as large as the continental United States; others argue for sudden changes in ocean levels and chemistry, including burps of methane gas; and still others cite the impact of an extraterrestrial object, similar to what caused the dinosaurs extinction.