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 expeditions of the Rus'
The Rus' launched the first large-scale raid in 913. A fleet of 500 ships reached the southern shores of the Caspian Sea through the country of the Khazars. In order to secure a peaceful passage through the land of the Khazars, the Rus' promised the Khazars half of their spoils. They sailed down the Dnieper River into the Black Sea, then into the Sea of Azov, then up the Don River past the Khazar city of Sarkel, and then by a portage reached the Volga, which led them into the Caspian Sea.