Sea level is the level of the ocean's surface. It changes continuously with the tides, and it varies from one part of the world to another. Mean sea level is the average sea level calculated from measurements made over a long period of time. Geographers use mean sea level as the starting level for measuring the height or depth of the earth's surface. For example, Mount McKinley in Alaska rises 20,320 feet (6,194 meters) above sea level. Death Valley, in California, is 282 feet (86 meters) below sea level.