Into the Deep


A sperm whale eats a giant squid. (illustration)

Richard Ellis/Photo Researchers, Inc.

Earth's climate depends on the ocean: evaporation of ocean waters brings moisture to land areas and transfers energy to the atmosphere, where it helps drive weather systems. Further, ocean currents move heat from the equator to the poles. The ocean's enormous influence on climate hinges on its impressive size: together, the world's oceans contain over 1 billion cubic kilometers (over 240 million cubic miles) of water.

The ocean's average temperature is around 3.6ºC (38.5ºF), but fortunately for families headed to the beach, the surface is much warmer. In fact, since water density depends on both temperature and salt content, the ocean is divided into three layers of different temperatures and salinities.

  • A thin, warm surface layer that is well mixed by currents.
  • 20-200 meters (70–700 feet) deep
  • A transition layer where temperature and salinity change as surface and deep water layers slowly mix.
  • 500–900 meters (1,600–3,000 feet) deep
  • A deep layer of cold, dense water, which contains about 65 percent of all water in the ocean.
  • The ocean has an average depth of about 2,800 meters (9,200 feet). Some parts of the ocean are much deeper—the Mariana Trench in the western Pacific extends down more than 11,000 meters (over 36,000 feet).