How did more than 75 whale fossils turn up in the middle of Chile's Atacama Desert — one of the world's driest? That's the question puzzling a team of scientists from the Smithsonian Institute, who discovered the "mysterious" whale graveyard over a half-mile away from the ocean. A guide to this surprising case:

What exactly did scientists find?

Dozens of ancient whale fossils, some at least as big as a bus, says the Associated Press. At least 20 "perfectly intact skeletons" were discovered. The bones were found side-by-side on a strip of road "the length of two football fields," suggesting that all of the whales died around the same time.

What kind of whales were they?

Among the remains — estimated to be between 2 million and 7 million years old — are adult and juvenile baleen whales, a walrus-whale, an extinct species of sperm whale, an extinct species of dolphin, and possibly the remains of a seal or sea lion. The site is "remarkable" for its unparalleled "diversity," says Nature.

How did the animals end up there?

It's unclear. Some theorists suggest that a mass beaching — in which whales swim onto land and die there — occurred millions of years ago when the desert wasn't as far from the Pacific Ocean as it is today. More likely, says lead Smithsonian researcher Nicholas Pyenson, the area was once a "lagoon-like environment" that somehow became closed off from the open ocean. The whales either died when the lagoon dried up, or when a natural disaster (such as an earthquake or landslide) sealed them off from the sea.

Have scientists found other whale graveyards?

Yes, in areas like Peru and Egypt. The specimens found in the Atacama Desert, however, are unique because of their "staggering number and beautifully preserved bones," says the AP.

Sources: Associated Press, Nature, Third Age