When you talk about mass extinctions, you're not talking recent history. You're talking geological history. But this is, without a doubt, the worst way history has repeated itself. Why? Mass extinctions occur when more than 75 percent of all species on Earth die off in a relatively short period [source: Cosmos]. It's happened five times over the past 500 million years, and some scientists think we're in the early throes of the sixth.
One of the most notable of the "Big Five" extinctions, as they are known, is the Cretaceous-Tertiary event. It happened about 66 million years ago, when volcanic activity, climate change and an asteroid impact wiped out 76 percent of all species, including the dinosaurs. But the granddaddy of them all was the Permian mass extinction: A massive eruption in Siberia sparked a catastrophic chain reaction that killed an incredible 96 percent of all species [source: Cosmos].
Now some scientists claim the sixth mass extinction is already underway. That's because species are going extinct at a much faster rate than normal — possibly three to 12 times faster. That gives us three to 22 centuries to do something before we reach mass extinction territory, which is certainly something we don't want to repeat! [source: Pappas]