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The Black Sea is an inland sea between southeastern Europe and Asia Minor. It is connected to the Mediterranean Sea by the Bosporus and the Sea of Marmara, to the Sea of Azov by the Strait of Kerch. There is a net inflow of seawater through the Bosporus, 200km per year. There is an inflow of freshwater from the surrounding areas, especially central and middle-eastern Europe, totalling 320km per year. The most important river entering the Black Sea is the Danube.
The Black Sea region is thought to have been the original homeland of "Proto-Indo-European", the progenitor of the Indo-European language family, by some scholars. Others move the heartland further east towards the Caspian Sea.
In 1997, William Ryan and colleagues from Columbia University published evidence that a massive flood through the Bosporus occurred about 5600 BC. They wrote: "Ten cubic miles of water poured through each day, two hundred times what flows over Niagara Falls. ... The Bosporus flume roared and surged at full spate for at least three hundred days." The event flooded 60,000 square miles of land, and significantly expanded the Black Sea shoreline to the north and east. The Black Sea's water level raised many hundreds of feet, and it was transformed from a fresh-water landlocked lake into a salt water sea connected to the ocean. It has been popularly suggested that this event was the source of the legend for Noah's Flood. Forcing a match requires that considerable license be taken with the Biblical story, however.
The Black Sea is the largest anoxic, or oxygen-free, marine system. This is a result of the great depth of the sea and the relatively low salinity (and therefore density) of the water flowing into it from rivers and the Mediterranean; freshwater and seawater mixing is limited to the uppermost 100-150m, with the water below this interface (called the pycnocline) being exchanged only once every thousand years. There is therefore no significant gas exchange with the surface, and as a result decaying organic matter in the sediment consumes any available oxygen. In these anoxic conditions some extremophile microorganisms are able to use sulfate (SO4) for oxidation of organic material, producing hydrogen sulfide (HS) and carbon dioxide. This mix is extremely toxic (a lungful would be fatal to a human), resulting in a sea that has almost all of its ecology living in that top layer down to a depth of approximately 500 feet -- for the rest of the over 7000 feet of depth, basically no life at all.
Large amounts of organic material reach the bottom of the sea and accumulate in the sediments in concentrations of up to 20%. These kinds of sediments are called sapropel.
The name (initially Pontus Euxinus) was coined by the Ancient Greek navigators, because of the unusual dark colour, compared with the Mediterranean Sea. Visibility in the Black Sea is on average approximately 15 feet (as compared to up to 100 feet in the Mediterranean).
Countries bordering on the Black Sea are Turkey, Bulgaria, Romania, Ukraine, Russia, Georgia, Crimea, an Autonomous Republic of Ukraine.
The most important cities along the coast are: Istanbul (formerly Constantinople and Byzantium) -- Burgas -- Varna -- Constanţa -- Odessa -- Sevastopol