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.
Dead Kultuk
It had a distinct coastline in former times, but since the 1990s, with higher Caspian Sea levels, the water penetrates inland through the neck of the bay producing waterlogged marshes. Located at the mainland end of the bay, the Kaydak Inlet cuts deep into the coast extending east and then southwards. Nowadays both the bay and the inlet are filled with Caspian Sea water. Currently there are oil fields in the area.
Owing to its special colour the Dead Kultuk is the bay which appears in early maps of the Caspian Sea as 'Blue Sea' ( in maps in that language). The area was mapped by Fedor Ivanovich Soimonov during the Caspian Expedition, which surveyed the Caspian Sea from 1719 to 1727, but was not accurately described until G. S. Karelin did so in 1832.