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 Kazakhstan
By contrast, the water level of the Caspian Sea has been rising steadily since 1978 for reasons that scientists have not been able to explain fully. At the northern end of the sea, more than 10,000 square kilometres of land in Atyrau Province have been flooded. Experts estimate that if current rates of increase persist, the coastal city of Atyrau, eighty-eight other population centers, and many of Kazakhstan's Caspian oil fields could be submerged by 2020.