A tsunami is a series of waves of extremely long length and period typically caused by a sudden vertical displacement of a large area of the sea floor during an undersea earthquake. The waves travel outward in all directions from the generating area, traveling at speeds of 300 to 600 miles per hour in the open ocean. Wave crests may be as much as 300 to 400 miles apart. In deep water, the height of the waves may be no more than 1 to 2 feet and may pass a surface vessel unnoticed. Upon reaching shallow water around islands or on a continental shelf, the speed of the advancing wave diminishes and its height increases greatly. Successive wave crests, each typically arriving from 10 to 45 minutes later, may continue to pound the coast for several hours.