 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.