Earthquakes also occur under the ocean. Sometimes, massive earthquakes that start on the ocean floor can create giant, destructive waves called tsunamis. These waves move at great speeds (up to about 800 kilometres or 500 miles per hour) and can travel thousands of kilometres across the ocean. In deep water, the waves are not very high. They gain strength and height as they approach the shore. Tsunamis can be about 30 metres (98 feet) high. These huge waves break on to the shore with a great deal of force, bringing down trees and large buildings.