May 14, 2002 —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 and branching chain of mountains snakes around the entire globe.

Deep diving submarines, including the Alvin, first studied the The Mid-Ocean Ridge system up close in the 1970s. The lava formations and hydrothermal vents that were found showed that the undersea mountains are in fact volcanoes, which are constantly creating the earth's new crust with hot magma flowing up from the planet's interior.

It was final confirmation of the idea that the earth's crust is always on the move. The once-controversial science of plate tectonics, which had been developing over many years, had come of age.

Click here for an illustrated tour of plate tectonics, sea floor spreading and continental drift.