Caspian Sea
The Caspian has characteristics common to both seas and lakes. It is often listed as the world's largest lake, although it is not a freshwater lake. It contains about 3.5 times more water, by volume, than all five of North America's Great Lakes combined. The Caspian was once part of the Tethys Ocean, but became landlocked about 5.5 million years ago due to plate tectonics. The Volga River (about 80% of the inflow) and the Ural River discharge into the Caspian Sea, but it has no natural outflow other than by evaporation. Thus the Caspian ecosystem is a closed basin, with its own sea level history that is independent of the eustatic level of the world's oceans.
Caspian lowland desert
The Caspian lowland desert ecoregion (WWF ID:PA1308) is an ecoregion that covers the north and southeast coasts of the Caspian Sea, including the deltas of the Volga River and Ural River in the northern region. While the region gets relatively low amounts of precipitation (less than 200 mm/year), wildlife is supported by the river estuaries and the sea itself. The wetlands are an internationally important area for bird nesting and migratory resting. The ecoregion is in the Palearctic ecozone, and the Deserts and Xeric Shrublands biome. It has an area of .