NOVA

(mysterious music) - [Narrator] Across more than 75 square miles, fossils litter the desert.

There are so many Hisham has to remove his shoes to avoid crushing them.

- This is typically what you see in Wadi Hitan.

The bone is sticking out from the cliff, calling you to come and see it.

You can find ribs all over the place.

Giving the size of the vertebrae, it might be getting up to 20 meter long.

- [Narrator] The size and shape of an animal around 60 feet long encased in this rock has led scientists to a remarkable conclusion.

- I think we have a complete skeleton of the prehistoric whale that lived here in Egypt long, long, long time ago.

(calm music) - [Narrator] This is the biggest ancient whale graveyard known on Earth.

That's why paleontologists named it Wadi Hitan, the Valley of the Whales.

But what are these sea creatures doing here in a desert over a hundred miles from the coast?

It's clear this area was once underwater.

(intriguing music) Today's excavation site was at the bottom of the sea 40 million years ago.

Back then, the Mediterranean was part of a much larger ocean, the Tethys.

It stretched from Europe to India and was full of marine life.

But when sea levels dropped, they left behind a seabed rich in fossils.

Today, this desert is the resting place of some of the earliest whales ever found.

They may hold the key to how today's ocean giants evolved.