Wanderlust World Wanderlust World The Caspian Sea is the remnant of an ancient body of water known as the Thetis Ocean. About 50 to 60 million years ago, the Thetis Ocean was connected to what are today the Atlantic and Pacific Oceans. As the continents drifted apart, the Thetis became isolated from those oceans and over time shrank to its present size. The Caspian's history as part of a large saltwater ocean is why it is one of the few saltwater lakes on Earth today.
The Caspian Sea is divided into three distinct regions: the Northern, the Middle and the Southern. The Northern Caspian accounts for 25 percent of the sea's surface area, but only 0.5 percent of its total volume due to its very shallow average depth. The Middle Caspian is home to a large 500-meter-deep underwater basin known as the Derbent Depression. In the Southern Caspian, which holds nearly two thirds of the Caspian's total volume, depths can reach over 1 km below the surface.
The manner in which the Caspian Sea became an isolated body of water accounts for the unique nature of many of its native species. The Caspian is home to 115 different species of fish, but is especially well known for the seven species of sturgeon that live there. Some of these sturgeon species existed 200 million years ago in many ancient bodies of water, but are today unique to the Caspian. In recent years, the Don-Volga Canal has allowed sturgeon from the Black Sea to become hybridized with native Caspian sturgeon, leading to a potential loss of biodiversity.
Pollution is a major problem in the Caspian Sea, though the levels have declined since the late 1980s. In the Northern Caspian, Russia's Volga River is the primary source of pollution. The many oil fields and refineries in or near the sea are major contributors as well. In addition to oil, heavy metals such as lead and cadmium are cause for concern in the Caspian. These materials can accumulate in the livers and reproductive organs of fish and other animals, leading to many heath problems.