The Caspian Sea is the world’s largest enclosed inland body of water.

Its defining feature is enclosure, not freshwater. Rivers enter a basin surrounded by land, while evaporation removes water without providing an oceanward escape route for most dissolved salts.

The Volga is its dominant freshwater supplier. The European Space Agency estimates that this river contributes about 80 percent of freshwater inflow, making conditions far upstream important to the Caspian's water balance.

A familiar figure for its surface area is about 371,000 square kilometers, but that is not a permanent surveyed boundary. Changes in water level alter the shoreline, especially across the shallow northern basin.

Russia, Kazakhstan, Turkmenistan, Iran and Azerbaijan share the Caspian shoreline. The water stretches roughly 1,200 kilometers from north to south in a depression between Europe and Asia. Its surface lies below global ocean level, but that does not connect it to the ocean: elevation and drainage are separate geographical properties. Water still requires an actual channel across the surrounding terrain to flow elsewhere.

The basin is far from uniform. Its northern portion is shallow and strongly influenced by incoming river water and sediment. The central and southern portions occupy much deeper depressions, with the southern basin reaching more than 1,000 meters in depth. Consequently, a map showing only the outline conceals major differences in water volume, circulation and the sensitivity of particular coastlines to falling levels.

ESA describes Caspian water as approximately one third as salty as ordinary seawater. That comparison is a broad guide rather than a measurement for every beach. Freshwater entering near river mouths dilutes the northern waters, while evaporation and restricted circulation can produce much saltier conditions in enclosed eastern bays. Calling the Caspian either freshwater or uniformly ocean-salty would miss this variation.

Its salt has a geological history as well as a modern water-balance explanation. ESA identifies the basin as a remnant of the ancient Tethys Sea. Today's rivers continue to deliver water and dissolved material, while evaporation transfers water back to the atmosphere. The present lake therefore reflects both inherited conditions and ongoing exchanges with its enormous catchment.

Enclosure also has ecological consequences. The Caspian supports distinctive plants and animals, but pollutants arriving from agriculture and industry do not simply flush into an open ocean. River management, water use and climatic changes can affect the amount of water reaching the basin. When discussing its size, a dated measurement is more informative than treating the familiar area figure as unchanging.

The Caspian’s name preserves a sea identity, but its physical enclosure makes it central to discussions of inland water. Its shores do not connect naturally to the world ocean. This matters both to its water balance and to regional definitions of landlocked countries. A coastal-looking landscape does not automatically provide the kind of ocean access implied by maritime geography.