Caspian Sea

The Caspian Sea lies east of the Caucasus Mountains and west of the Central Asian steppe. Its shores are shared by five countries: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. Although it is called a sea, it is an enclosed inland body of water, separated from the world ocean. It is commonly described as the world’s largest inland body of water and largest salt lake. The basin is divided into northern, middle, and southern sections: the north is shallow, while the southern basin contains the greatest depths. Its surface lies below global sea level. The Caspian has not always been isolated; geological evidence indicates that its basin was connected to nearby seas at earlier points in Earth’s history.

The Caspian has no natural outlet to the ocean, so its level reflects the balance among river inflow, precipitation, and evaporation. The Volga is its largest river source; the Volga and Ural together account for more than four-fifths of inflowing water in the estimates cited by recent research. Water levels have varied over historical periods, and changes affect coastlines, wetlands, ports, and other infrastructure. A 2026 study in Earth’s Future, analyzing satellite observations, river records, and climate data for 1991–2020, found a significant decline in total river inflow, primarily associated with lower discharge from the Volga; it also reported broadly stable basin precipitation and a modest upward trend in evaporation. A separate 2025 geospatial study modeled possible future water-level declines under global warming and assessed resulting risks to habitats, protected areas, and infrastructure. Its projections are scenario-dependent rather than a certain forecast.

The basin’s long isolation has produced distinctive ecosystems. The United Nations Environment Programme describes more than 400 species as endemic to the Caspian, including the Caspian sturgeon and the rare freshwater Caspian seal. River systems and wetlands also support large numbers of migrating birds. Fishing, shipping, and petroleum and natural-gas production are important to the regional economy. These activities bring environmental pressures: UNEP identifies pollution associated with oil extraction and refining, offshore fields, sewage, and industrial waste as concerns for the marine environment.

Because the sea and its rivers cross national boundaries, environmental protection requires coordination among the five littoral states. The Tehran Convention, signed in 2003, was the first legally binding regional agreement signed by all five countries for protecting the Caspian’s marine environment. UNEP hosts its interim secretariat. The convention provides a framework for cooperation, with protocols addressing subjects such as biodiversity, environmental assessment, land-based pollution, and oil spills.