The Black Sea is an enclosed inland sea, connected to the Mediterranean Sea via the Dardanelles, the Sea of Marmara, and the Bosporus. There is negligible exchange of water with the Mediterranean, and the surface waters of the Black Sea are about half as saline as the eastern Mediterranean. A previous small outflow through the Bosporus to the Aegean appears to have been reversed due to reduced inflow after the damming of some of the rivers feeding the Black Sea.
Although the surface waters are relatively fresh, below about 330–490 ft (100–150 m) lies a highly saline water body with very slow turnover. Decaying organic matter consumes all the oxygen in this water, making the Black Sea the world’s largest oxygen-free marine system—the deep water is essentially dead. The basin is an isolated remnant of the north shore of the ancient Tethys Ocean. The southern part of the Black Sea is deep, but it is not as deep as the Mediterranean, and the underlying crust is thicker than most ocean crust.
The northern parts—the Sea of Azov and the Gulf of Odessa—overlie a shallow continental shelf. The delta of the Danube, Europe’s longest river, extends from the western shore, and Danube waters have carried sediment across the edge of the shelf to build up a thick cone of sediment.