Megalodon

Megalodon makes a great monster for Shark Week and scary movies, even though they died out millions of years ago. These sharks could get as big as a tennis court! And since we marvel at how vast and how deep the oceans are, and how we've never been able to explore most of it because it's just that big, we can tell ourselves that there may be a megalodon or two lurking where we cannot see, just like colossal squids. But they really really are extinct. Besides that, a megalodon wouldn't even bother with eating humans, because they needed substantial meals, like whales.

Shark scientist Jaida Elcock of MIT addresses the question of how and why megalodon became the largest shark ever. How they can tell so much about an extinct fish just from the teeth they left behind is fascinating. There's a one-minute skippable ad at 4:37