Much of the seafloor is mountainous and uneven, even more so than the earth's surface. As oceanographers first began mapping the ocean bottom, they discovered that often near the center of ocean basins the seafloor is dominated by a linear mountain chain, or mid-oceanic ridge with vast rising slopes. During World War II, oceanographer William Maurice Ewing began mapping the complex ocean bottom with sophisticated instruments such as sonar depth finders and underwater cameras that helped trace the contours of the ocean bottom. Ewing set out to measure and record a massive chain of undersea mountains called the Mid-Atlantic Ridge. When Ewing and his crew began mapping the massive ridge, they encountered a problem: the sonar beams were bouncing back. This problem led to an interesting discovery--they realized that there were frequent oceanic earthquakes occurring along the ridge.

This was an exciting...