During the 1950 and 1960s, three decades after Wegener’s death, scientists started to reexamine continental drift due to advances in marine sonar technology. This equipment produced detailed maps of the ocean floor and the unexpected discovery of massive ridges under the sea. These submerged mountain chains were created by hot magma-liquid rock spouting up from beneath Earth’s crust. As it cooled and hardened, the magma became new crust, and at a speed of about four inches per year, this crust traveled away from either side of the ridge.