The Black Sea is connected to the distant waters of the Atlantic Ocean by the succession of the Bosporus (a strait at the Black Sea's southwestern corner), the Sea of Marmara, the Dardanelles, the Aegean Sea, and the Mediterranean Sea. The peninsula of Crimea extends into the sea from the north, and immediately to the east the narrow Kerch Strait opens onto the smaller Sea of Azov. The Black Sea's water-surface area is about 178,000 square miles (461,000 square km), and its maximum depth is more than 7,250 feet (2,210 m). The Black Sea has few coastal lowlands. The Danube, Dnieper, Dniester, and Don are the largest rivers emptying into the sea.
The Black Sea is a residual basin of the ancient Tethys Sea; its present form probably emerged about 58 million years ago when structural upheavals in Anatolia split off the Caspian basin from the Mediterranean. The newly formed Black Sea basin gradually became isolated from the ocean, its salinity was reduced, and it was slowly separated from the Caspian region. The salinity of the Black Sea is almost half that of the world's oceans.
An unusual feature of the Black Sea is that oxygen is dissolved only in the upper levels of its waters, which alone can support a rich sea life as a result. Below a depth of 230-330 feet (70-100 m) at the centre and 330-500 feet (100-150 m) near the sea's margins, there is no oxygen because the sea is permeated by a high concentration of dissolved hydrogen sulfide, forming a saturated, "dead" zone inhabitable only by specially adapted bacteria. Despite this anomaly, the Black Sea's uppermost waters supported abundant sturgeon, mackerel, and anchovy until the late 20th century, when the runoff of industrial and municipal wastes into the Danube, Dnieper, and other feeder rivers caused increasing levels of pollution and consequent reductions in fish populations.
The Black Sea remains an important shipping artery linking Ukraine, Bulgaria, Romania, and southwestern Russia with world markets. The sea's northern coast, particularly the Crimea, is a major recreational area for eastern European vacationers.
