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.
Russian conquest of Central Asia
The area was bounded on the west by the Caspian Sea, on the north by the Siberian forests and on the east by the mountains along the former Sino-Soviet border. The southern border was political rather than natural. It was about 1300 miles from north to south, 1500 miles wide in the north and 900 miles wide in the south. Because the southeast corner (Kyrgyzstan and Tajikistan) is mountainous the flat desert-steppe country is only about 700 miles wide in the south. Using modern borders, the area was 1,545,730 square miles, about half the size of the United States without Alaska. On the east side two mountain ranges project into the desert. Between them is the well-populated Ferghana Valley which is approximately the 'notch' on the west side of Kyrgyzstan. North of this projection the mountain-steppe boundary extends along the north border of Kyrgyzstan about 400 miles before the mountains turn north again.