The ocean floor is mountainous and uneven, much like Earth's surface. As oceanographers began mapping the ocean bottom, they discovered that the sea floor is full of vast rising slopes, or ridges, and dramatic open valleys, or rifts. 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 Midatlantic 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 another great discovery. They realized that there were frequent oceanic earthquakes occurring along the ridge. This was an exciting discovery because it opened up the possibility that oceanic earthquakes...