I know, I know. The answer is usually "a bigass meteor". But Princeton geoscientist Gerta Keller has a new theory that is pretty darn convincing.
Keller found that the meteor
we think killed the dinosaurs really didn't cause the mass extinction event attributed to it Analysis of the fossil record show that a significant number of late-Cretaceous species persisted for 300,000 years after the impact. Proponents of the asteroid theory usually explain this away by suggesting the fossil record was jumbled by the meteor itself. Except Keller found zero evidence of any geological trauma, even in Mexico where the meteor impact occurred. So what killed the dinosaurs?
Keller believes that massive - and sustained - volcanic eruptions in India did the job. The massive ash clouds would have blocked out the sun, created a deluge of acid rain, and generally made life pretty impossible for giant lizards.
So here's the good news: massive meeor impacts may not wipe us all out (only the people under or near the death rock). On the other hand, if enough volcanoes start erupting, we're screwed.