Bosporus, a strait in Turkey. Together with the Sea of Marmara and the Dardanelles strait it links the Black Sea with the Mediterranean Sea and separates European and Asian Turkey. The Bosporus is about 18 miles (29 km) long and one-half to two miles (800 to 3,200 m) wide; it is deep enough for the largest ships. Along its banks stand old fortresses. Istanbul lies at the southern end of the Bosporus. The strait is spanned by one of the world's longest suspension bridges, opened in 1973.
As the only outlet from the Black Sea, and situated across the major land route between Europe and Asia, the Bosporus has long been of great strategic value. Under the terms of the Montreux Convention of 1986, merchant vessels of all nations have freedom of passage through the Bosporus and the Dardanelles. Turkey gained the right to fortify the straits and to close them in time of war or threat of war.