If the movie Jaws scared you away from swimming, perhaps you should avoid the "Journey through Time" section of the Sant Ocean Hall at the National Museum of Natural History. There you'll find a collection of fossil marine life dating back as far as 500 million years ago. In one case is possibly the scariest item in the place—the jaw of a giant great white shark, Carcharadon megalodon, opened wide enough to take in a few adult humans at once and with rows of teeth as big as my hand. It's no wonder that these fossils have inspired a series of bad sci-fi films.

We can all sleep easy, though; megalodon lived 25 to 1.5 million years ago and is long gone from today's oceans.

Megalodon was the world's largest shark, growing to 60 or 70 feet in length and 77 tons in weight. It roamed warm oceans (fossils have been found all over the world) eating around 2,500 pounds of food each day, scientists have estimated, including fish and whales. One 2008 study calculated that this giant shark had a bite force of 12 to 20 tons, about 6 to 10 times that of modern great whites.

What led to their demise? Scientists aren't sure, but the chief suspect is shrinking habitat. When this shark lived, the world was forming into the one we now recognize—the Himalayas and Rockies were growing, the Isthmus of Panama rose from the sea to separate the Atlantic and Pacific, then massive glaciation locked much of the world's water in ice. Everything was changing for the big sharks, possibly including what they ate and where they raised their kids, and they just couldn't survive in the new world.

Rumors of megalodon's survival persist on the Internet. But no live specimen, or even fresh teeth, has ever been found, making it pretty unlikely that this shark still exists.