A remnant of the Paratethys Sea, the Caspian Sea is the largest inland body of water on Earth. Covering more than 143,000 square miles, it was cut off from the Black Sea when the Caucasus Mountains rose some 6 million years ago, a product of the Red Sea opening which forced (and continues to force) the Arabian Plate to collide with the Eurasian Plate. Retaining some salinity from the Paratethys, the Caspian Sea has become a massive basin lake, receiving inflow from more than 130 freshwater rivers but having no outlet to the sea; 80% of the inflow is via the Volga River while the combined flow of the Ural and Kura Rivers adds another 15%.
Since this inflow is primarily from the north and northwest shores of the Caspian Sea, its relatively shallow northern waters (only 20 feet deep) are fresh while the deeper waters of its southern section (exceeding 3000 feet in some areas) have a salinity that is 1/3 of that found in our oceans. Kara-Bogaz-Gol, a large lagoon on its eastern shore (within Turkmenistan) has been nearly cutoff by sandbars and receives inflow only during periods of high water in the Caspian Sea; the lagoon's waters, subject to intense evaporation, have a salinity that is 10 times that of sea water. Today, five countries border the Caspian Sea: Russia on the northwest, Kazakhstan on the northeast, Azerbaijan on the southwest, Turkmenistan on the southeast and Iran on the south; except for the Caspian Depression to its north, the Caspian Sea is rimmed by highlands and its surface elevation, while variable, is about 92 feet below sea level.
While lush wetlands are spaced along its western and southern shores, the Caspian Sea is bordered by desert on its northern and eastern coasts; of special interest is a region of geothermal mud domes along part of the Azerbaijan coast, a testament to plate subduction in that area. Famous for a magnificent diversity of birdlife (especially during migrations), its prized sturgeon caviar and its endemic Caspian seals (thought to have arrived when the Paratethys connected to the North Sea), this unique and varied ecosystem is threatened by both global warming and industrial pollution (especially from oil and gas production). Since it is rimmed by five countries, the effort to protect this ecological treasure is especially challenging.
