In Egypt's Sahara Desert, massive skeletons with strange skulls and gigantic teeth jut out from the sandy ground. This fossil graveyard, millions of years old, is known as the "Valley of the Whales."  Now, paleontologists have unearthed a whole new species of ancient whale dating to 43 million years ago, and this predator wasn't just able to swim – it also had four legs and could walk. Follow scientists as they search for new clues to the winding evolutionary path of mammals that moved from the land into the sea to become the largest animals on Earth.