In the 1950s, scientists using depth-sounding surveys discovered an enormous undersea mountain range called the Mid-Ocean Ridge. Measuring 42,000 miles long and up to 15,000 feet high, this winding chain was first studied up close by Bob Ballard in the 1970s. Exploring in the submersible Alvin, Ballard found undersea lava formations and, later, hydrothermal vents. His discoveries help prove that this series of undersea volcanoes was responsible for creating the surface crust of the entire planet.

At the time of Ballard's discoveries, the body of evidence on the Mid-Ocean Ridges and their role in the Earth's formation had been mounting for decades.

Click here for an illustrated tour of plate tectonics and continental drift.