Caspian Sea [key], Lat. Mare Caspium or Mare Hyrcanium, salt lake, c.144,000 sq mi (373,000 sq km), between Europe and Asia; the largest lake in the world. It is bordered on the northeast by Kazakhstan, on the southeast by Turkmenistan, on the south by Iran, on the southwest by Azerbaijan, and on the northwest by Russia. The Caspian's surface lies 92 ft (28 m) below sea level. It reaches its maximum depth, c.3,200 ft (980 m), in the south; the shallow northern half averages only about 17 ft (5 m). The Caucasus Mts. rise from the southwestern shore, and the Elburz Mts. parallel the southern coast. The Caspian receives the Volga (which supplies more than 75% of its inflow), Ural, Emba, Kura, and Terek rivers, but has no outlet. The rate of evaporation is particularly high in the eastern inlet called Garabogazkol, which is exploited for salt. Variations in evaporation account for great changes in the size of the sea during the course of history. The damming and diversion of the Volga's water for industrial and residential use have been the leading reasons for the lowering of the Caspian's water level, a problem of serious proportions, but increases in evaporation rates and decreases in precipitation due to climate change are expected to accelerate the drop in the water level during the 21st cent. The chief ports on the Caspian are Bakı, a major oil center, and Astrakhan, at the mouth of the Volga.
Underlying the Caspian are some of the world's largest oil reserves, and the five surrounding countries, all with major stakes in oil-field development, have disputed zones of control, although Russia has signed territorial agreements with Kazakhstan and Azerbaijan. The Caspian also has important fisheries. The northern part of the sea is the chief source of beluga caviar, but the destruction of spawning areas and illegal fishing has greatly reduced the number of sturgeon, and fishing quotas have been imposed. In 2003 a framework treaty for the protection of the sea's environment was signed by four of the surrounding nations; Turkmenistan did not sign. The Convention on the Legal Status of the Caspian Sea, signed in 2018 by the bordering nations, established different guidelines for the division of the surface waters, which are divided into territorial waters (15 nautical mi/28.8 km), fishing waters (an additional 10 nautical mi/18.5 km), and common waters, and the seabed, which is divided into exclusive zones determined by bilateral agreements.
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