Bosporus

Strait separating the European and Asian portions of Turkey. Connecting the Sea of Marmara with the Black Sea, it is 19 mi (31 km) long and 2.3 mi (3.7 km) at its widest. Bosporus literally means “ox ford”; it is traditionally connected with the legendary figure of Io, who in the form of a heifer crossed the Thracian Bosporus in her wanderings.

Because of its strategic importance for the defense of Constantinople (modern Istanbul), which straddled its southern end, the Byzantine emperors and later the Ottoman sultans constructed fortifications along its shores. With the growing influence of the European powers in the 19th century, rules were codified governing the transit of vessels through the strait. An international commission assumed control of it after World War I; Turkey resumed control in 1936. Two of the world's longest bridges, completed in 1973 and 1988, span the strait and link the two sections of Istanbul; in 2004 construction began on a rail tunnel beneath it.