The Bosporus (Greek: Βόσπορος) is a strait that separates the European part (Rumeli) of Turkey from its Asian part (Anatolia), connecting the Sea of Marmara (Turkish: Marmara Denizi, Greek: Θάλασσα του Μαρμαρά) with the Black Sea (Turkish: Karadeniz, Greek: Μαύρη Θάλασσα, Εύξεινος Πόντος). It is 30 km long, with a maximum width of 3,700 metres at the northern entrance, and a minimum width of 750 metres between Anadoluhisarı and Rumelihisarı. The depth varies from 36 to 124 metres in midstream.
The shores of the strait are heavily populated as the city of Istanbul (population at least 11 million) straddles it.
Two bridges cross the Bosporus Strait. The first, Bosporus Bridge, is 1074 meters long and was completed in 1973. The second, Fatih Sultan Mehmed (Bosporus II) bridge, is 1090 meters long, and was completed in 1988 about five kilometers north of the first bridge.
"Marmaray", a 13.7 kilometer-long rail tunnel is under construction and expected to be completed in 2008. Approximately 1,400 metres of the tunnel will run under the strait, at a depth of about 55 meters.
It is also said in myth that floating rocks once crushed any ship that attempted passage of the Bosporus until the hero Jason obtained passage by trickery, whereupon the rocks became fixed, and Greek access to the Black Sea was opened.
The Bosporus formed about 5600 BC when the rising waters of the Mediterranean/Sea of Marmara breached through to the Black Sea, which at the time was a low-lying body of fresh water.
Some have argued that the resulting massive flooding of the inhabited and probably farmed northern shores of the Black Sea is the historic basis for the flood stories in the Epic of Gilgamesh and the Bible.
As the narrowest point of passage between the Black Sea and the Mediterranean, the Bosporus has always been of great commercial and strategic importance. The Greek city-state of Athens in the fifth century BC, which was dependent on grain imports from Scythia, therefore maintained critical alliances with cities which controlled the straits, such as the Megarian colony Byzantium.
The strategic significance of the strait was one of the factors in the decision of the Roman Emperor Constantine the Great to found there in 330 AD his new capital, Constantinople. This city was in 1453 to become the capital of the Ottoman Empire, and as they closed in on the Byzantine city the Ottomans constructed a fortification on each side of the strait, Anadoluhisari (1393) and Rumelihisari (1451).
The strategic importance of the Bosporus remains high, and control over it has been an objective of a number of hostilities in modern history, notably the Russo-Turkish War, 1877-1878, as well as of the attack of the Allied Powers on the Dardanelles in 1915 in the course of the First World War. Several international treaties have governed vessels using the waters, including the Montreux Convention Regarding the Regime of the Turkish Straits, signed in 1936.