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.
Lake Superior
Lake Superior empties into Lake Huron via the St. Marys River and the Soo Locks. Lake Superior is the largest freshwater lake in the world in area (if Lakes Michigan and Huron are taken separately; "see Lake Michigan–Huron"), and the third largest in volume, behind Lake Baikal in Siberia and Lake Tanganyika in East Africa. The Caspian Sea, while larger than Lake Superior in both surface area and volume, is brackish; though presently isolated, prehistorically the Caspian has been repeatedly connected to and isolated from the Mediterranean via the Black Sea.