One of the major subdivisions of the interconnected body of salt water that occupies almost three-quarters of the Earth's surface. Earth is the only planet in the solar system whose surface is covered with significant quantities of water. Of the nearly 1.4 billion cubic kilometers of water found either on the surface or in relatively accessible underground supplies, more than 97% is in the oceans; most of the rest is in glacier-covered Greenland and Antarctica (Table 1). During much of Earth's history the oceans have been a difficult, if not quite impassable, barrier for the movement of land-based plants and animals from one continent or island to another. This article describes and compares some of the major oceans and their features.  See also: Glaciology; Oceanography