It is approximately 30 km long, with a maximum width of 3,700 metres at the northern entrance, and a minimum width of 700 metres between Kandilli and Asiyan; and 750 metres between Anadoluhisari and Rumelihisari. The depth varies from 36 to 124 metres in midstream.
The world's narrowest strait used for international navigation, it connects the Black Sea with the Sea of Marmara (which is connected by the Dardanelles to the Aegean Sea, and thereby to the Mediterranean Sea).
The shores of the strait are heavily populated as the city of Istanbul (with a metropolitan area in excess of 11 million inhabitants) straddles it.
Two bridges cross the Bosporus. The first, the Bosphorus Bridge, is 1074 metres long and was completed in 1973. The second, Fatih Sultan Mehmet (Bosphorus II) bridge, is 1090 metres long, and was completed in 1988 about five kilometres north of the first bridge. A third road bridge is also being planned for one of seven locations designated by the Turkish Government. The location is being kept secret to avoid an early explosion in land prices.
Another crossing, Marmaray, is a 13.7 kilometre-long rail tunnel currently 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 metres
The name comes from the Greek word Bosporos (Βόσπορος). Its etymology is from bous (βοῦς: ox) and poros (πόρος: passage, strait), thus meaning "oxen passage", which could reflect the older history of the region. The Greeks wrongly analysed it as "ox-ford" or "shallow sea ox passage" and associated it with the myth of Io's travels after Zeus turned her into an ox for her protection. It has also been thought to be a Thracian form of Phôsphoros, 'light-bearing', an epithet of the goddess Hecate.
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.
Formation of the Bosporus
The exact cause for the formation of the Bosporus remains the subject of vigorous debate among geologists. Thousands of years ago, the Black Sea became disconnected from the Aegean Sea. One recent theory (published in 1997 by William Ryan and Walter Pitman from Columbia University) contends that the Bosporus was formed about 5600 BC when the rising waters of the Mediterranean/Sea of Marmara breached through to the Black Sea, which at the time (according to the theory) 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 thought to be the historic basis for the flood stories found in the Epic of Gilgamesh and in the Bible in Genesis, Chapters 6-9. On the other hand, there is also evidence for a flood of water going in the opposite direction, from the Black Sea into the Sea of Marmara around 7000 or 8000 BCE.
Ancient Greece, Rome, the Byzantines and the Ottoman Empire
620 historic waterfront houses (yali) stretch along the coasts of the Bosphorus, such as the "yali" of Kibrisli Mehmed Emin Pasha (Mehmed Emin Pasha the Cypriot)St. Jerome's Vulgate translates the Hebrew besepharad in Obadiah, 1-20 as "Bosforus", but other translations give it as "Sepharad" (probably Sardis, but later identified with Spain).
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 5th 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, which came to be known as the capital of the Eastern Roman Empire. On May 29, 1453 it was conquered by the emerging Ottoman Empire. In fact, as the Ottoman Turks closed in on Constantinople, they constructed a fortification on each side of the strait, Anadoluhisari (1393) and Rumelihisari (1451). They later renamed the city Istanbul.
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.
The cheapest way to experience Bosphorus in Istanbul would be taking the public ferries that travel between the Anatolian and Rumelian sides of the city in every 45 minutes. It costs about 1 YTL (about 0.75 Euros). There are also faster ferries that take off in every 10 minutes, but the slower ones will give you more opportunity to watch the city. One can also take a ride on a variety of touristic ships from modern ones to Ottoman style ones.