The Bosporus or Bosphorus, also known as the Istanbul Strait, (Turkish: İstanbul Boğazı) (Greek: Βόσπορος) is a strait that forms the boundary between the European part (Rumelia) of Turkey and its Asian part (Anatolia). 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). 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 Aşiyan; and 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 (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. Plans for a third road bridge, which will allow transit traffic to by-pass the city traffic, have been approved by the Ministry of Transportation. The bridge will be part of the so called “Northern Marmara Motorway”, which will be further integrated with the existing Black Sea Coastal Highway. The location will be somewhere north of the existing two bridges, but the exact path is kept secret to avoid an early boom in land prices.

Another crossing, Marmaray, is a 13.7 kilometre-long undersea railway tunnel currently under construction and is expected to be completed in 2012. Approximately 1,400 metres of the tunnel will run under the strait, at a depth of about 55 metres.

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 BCE 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 Book of 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.

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