10. No one knows why Megalodon went extinct.
So Megalodon was huge, relentless, and the apex predator of the Pliocene and Miocene epochs. What went wrong? Well, there's no lack of theories: Megalodon may have been doomed by global cooling (which culminated in the last Ice Age), or by the gradual disappearance of the giant whales that constituted the bulk of its diet. (By the way, some people think Megalodons still lurk in the ocean's depths, as detailed in the Discovery Channel special Megalodon: The Monster Shark Lives, but there's absolutely no reputable evidence to support this. See this review for more about this made-up "documentary.")