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.
Geography of the Soviet Union
The Soviet Union's four million inland bodies of water chiefly resulted from extensive glaciation. Most prominently they included the Caspian Sea, the world's largest inland sea, and Lake Baikal, the world's deepest and most capacious freshwater lake. Lake Baikal alone held 85 percent of the freshwater resources of the lakes in the Soviet Union and 20 percent of the world's total. Other water resources included swampland - a sizable portion of territory (10 percent) - and glaciers in the northern areas.