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Bosfora or Босфора (Bulgarian), otherwise known as İstanbul Boğazı or Istanbul Strait or the Bosphorus or Bosporus (English), Boğaziçi (Turkish), or Βόσπορος or Vosporos (Greek), is a strait that forms part of the boundary between Europe and Asia. It is one of the Turkish Straits, along with the Dardanelles. 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.)

Bosphorus' limits are defined as the connecting line between the lighthouses Rumeli Feneri and Anadolu Feneri in the north and between the Ahırkapı Feneri and the Kadıköy İnciburnu Feneri in the south. The strait is between the limits 31 km (17 nmi) long, with a width of 3,329 m (1.798 nmi) at the northern entrance and 2,826 m (1.526 nmi) at the southern entrance. Its maximum width is 3,420 m (1.85 nmi) between Umuryeri and Büyükdere Limanı, and minimum width 700 m (0.38 nmi) between Kandilli Point and Aşiyan. A 45 degree course alteration is required for the ships at this point. The current can reach 7-8 knots at this point. It is a highly dangerous point for ships navigation. At Yenikoy, the necessary course alteration is 80 degrees. All the dangers and obstacles characteristic of narrow waterways are present and acute in this critical sea lane. At the above mentioned turns (Kandilli and Yenikoy) where significant course alterations have to be made, the rear and forward sights are totally blocked prior to and during the course alteration. The ships approaching from the opposite direction cannot be seen round these bends. There is also very heavy ferry traffic in the Strait of Istanbul, which crosses between European and Asiatic sides of the city.

The depth of Bosphorus varies from 36 to 124 m (118 to 407 ft) in midstream with an average of 65 m (213 ft). The deepest location is between Kandilli and Bebek with 110 m (360 ft). The most shallow locations are off Kadıköy İnciburnu on the northward route with 18 m (59 ft) and off Aşiyan Point on the southward route with 13 m (43 ft).

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.

The name comes from Greek Bosporos (Βόσπορος), which the ancient Greeks analysed as bous βοῦς 'ox' + poros πόρος 'means of passing a river, ford, ferry', thus meaning 'ox-ford'. Although it has been known for a while that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow, findings of a study by the University of Leeds in August 2010 reveal that there is in fact an underwater river flowing through the Mediterranean and under the Bosphorus (caused by the difference in density of the two seas), which would be the sixth largest river on Earth if it were to be on land.

It is site of several open water swims including the Bosphorus Cross-Continental Swim, International Bosphorus Swim (Boğazıçi), and the Bosphorus International Swim.

Bosphorus Cross-Continental Swim

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