Since the 1960s, mankind has dreamed of living under the waves. Some habitats were even built on the seafloor, but our yearning for the stars left them abandoned. Now, Fabien Cousteau, grandson of the most famous explorer of the deep, wants to build "an International space station” down there. Called Proteus, he hopes it will add to human knowledge not just of the seas, but the entire planet. Video by Tom Gibson