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The Bosphorus (//) or Bosporus (//; Ancient Greek: Βόσπορος, Bósporos; Turkish: Boğaziçi) is a strait that forms part of the boundary between Europe and Asia. The Bosporus, the Sea of Marmara, and the Dardanelles strait to the southwest together form the Turkish Straits. The world's narrowest strait used for international navigation, the Bosporus 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 limits of the Bosporus 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. Between the limits, the strait is 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. This part of the strait is a dangerous point for maritime traffic: a 45-degree course alteration is required, and the current can reach 7 to 8 knots (3.6 to 4.1 m/s). To the south, at Yeniköy, 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 Yeniköy) where significant course alterations have to be made, the rear and forward sights are totally blocked prior to and during the course alteration. Ships approaching from the opposite direction cannot be seen around these bends. The risks posed by geography are multiplied by the heavy ferry traffic across the strait, linking the European and Asian sides of the city.
The depth of the Bosporus varies from 13 to 110 m (43 to 361 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 Golden Horn is an estuary off the main straits that acted as a moat to protect Old Istanbul from attack, as well as providing a sheltered anchorage for the imperial navy until the 19th century.
Most of the shores of the strait are heavily populated, straddled as it is by the city of Istanbul (with a metropolitan area population in excess of 12 million inhabitants) which extends inland from both coasts.
It has been known since before the 20th century that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow and in August 2010 a continuous 'underwater channel' of suspension composition was discovered to flow along the floor of the Bosporus which would be the sixth largest river on Earth if it were to be on land. The study of the water and wind erosion of the straits relates to that of its formation. Sections of the shore have been reinforced with concrete or rubble and sections of the strait prone to deposition are periodically dredged.
The Bosporus is also known as "Strait of Constantinople", or as "Istanbul Strait" (Turkish: İstanbul Boğazı). To distinguish it from the Cimmerian Bosporus, it was anciently known as the Thracian Bosporus (Herodotus 4.83; Bosporus Thracius, Bosporus Thraciae , Βόσπορος Θρᾴκιος, also Chalcedonian Bosporus, Bosporus Chalcedoniae, Bosporos tes Khalkedonies, Herodotus 4.87, or Mysian Bosporus, Bosporus Mysius). The term could also be used as common noun βόσπορος, meaning "a strait", and was also applied to the Hellespont in Classical Greek (Aeschylus, Sophocles).
The Greek name Βόσπορος (Bosporos) was folk-etymologized as from βοὸς πόρος, i.e. "cattle strait" (or "Ox-ford"), from the genitive of bous βοῦς "ox, cattle" + poros πόρος "passage", thus "cattle-passage", or "cow passage" in reference to Io from Greek mythology who was transformed into a cow and condemned to wander the earth until she crossed the Bosporus where she met Prometheus. This folk etymology was canonized by Aeschylus in Prometheus Bound (v. 734f.), where Prometheus prophesies to Io that the strait would be named after her. The site where Io supposedly went ashore was near Chrysopolis, was named he Bous "the Cow". The same site was also known as Damalis, as it was where the Athenian general Chares had erected a monument to his wife Damalis. This monument included a colossal statue of a cow (the name Damalis translating to "calf"). The actual etymology of the name is more likely from the verb βύζω or βύω, "to fill up, clog up, plug, stop", referring to a "plugged" or stopped-up passage, perhaps also cognate with the name of Byzantium (Hesychius has βύζαντες: πλήθοντες, i.e. buzantes meaning "filled up").
The spelling with -ph-, as Bosphorus, has no justification in the ancient Greek name, but it occurs as a variant in medieval Latin (as Bosphorus, and occasionally Bosforus, Bosferus), and in medieval Greek sometimes as Βόσφορος, giving rise to the French form Bosphore, Spanish Bósforo, and Russian Босфор. The 12th-century Greek scholar John Tzetzes calls it Damaliten Bosporon (after Damalis), but he also reports that in popular usage the strait was known as Prosphorion during his day, the name of the most ancient northern harbour of Constantinople.
The exact cause for the formation of the Bosporus remains the subject of debate among geologists. Thousands of years ago, the Black Sea became disconnected from the Aegean Sea. The Black Sea deluge theory (reinforced in credibility by a study of the same name of 1997 by two scientists 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.
It is also said in myth that floating rocks known as the Symplegades or Clashing Rocks once crushed any ship that attempted passage of the Bosporus until the hero Jason obtained passage, whereupon the rocks became fixed, and Greek access to the Black Sea was opened.
As part of the only 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, maintained critical alliances with cities which controlled the straits, such as the Megarian colony Byzantium.
Persian King Darius I the Great, in an attempt to subdue the Scythian horsemen who roamed across the north of the Black Sea, crossed through the Bosporus, then marched towards the Danube River. His army crossed the Bosporus over an enormous bridge made by connecting Achaemenid boats. This bridge essentially connected the farthest geographic tip of Asia to Europe, encompassing at least some 1000 meters of open water if not more. Years later, a similar boat bridge would be constructed by Xerxes I on the Dardanelles (Hellespont) strait, during his invasion of Greece. Byzantines called the Bosporus "Stenon" and most important toponyms of it Bosporios Akra, Argyropolis, St. Mamas, St. Phokas, Hestiai or Michaelion, Phoneus, Anaplous or Sosthenion in European side and Hieron tower, Eirenaion, Anthemiou, Sophianai, Bithynian Chryspolis in Asian side in this era 
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 AD 330 his new capital, Constantinople, which came to be known as the capital of the Eastern Roman Empire. On 29 May 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, Anadoluhisarı (1393) and Rumelihisarı (1451).
The Bosporus remains strategically important. It is a major sea access route for Russia and Ukraine. Control over it has been an objective of a number of hostilities in modern history, notably the Russo-Turkish War (1877–78), as well as of the attack of the Allied Powers on the Dardanelles during the 1915 Battle of Gallipoli in the course of World War I.
At its peak in the 16th through the 18th centuries, the Ottoman Empire had wrested control of the entire Black Sea area, which was for the time an "Ottoman lake", on which Russian warships were prohibited.
Subsequently, several international treaties have governed vessels using the waters. Under the Treaty of Hünkar Iskelesi of 1833, the Bosporus and Dardanelles straits were to be closed on Russian demand to naval vessels of other powers. Following World War I, the 1920 Treaty of Sèvres demilitarized the strait and made it an international territory under the control of the League of Nations. This was amended under the 1923 Treaty of Lausanne, which restored the straits to Turkish territory—but allowed all foreign warships and commercial shipping to traverse the straits freely. Turkey eventually rejected the terms of that treaty, and subsequently Turkey remilitarized the straits area. The reversion was formalized under the Montreux Convention Regarding the Regime of the Turkish Straits of July 1936. That convention, which is still in force, treats the straits as an international shipping lane save that Turkey retains the right to restrict the naval traffic of non-Black Sea states.
Turkey was neutral in World War II until February 1945, and the straits were closed to the warships of belligerent nations during this time, although some German auxiliary vessels were permitted to transit. In diplomatic conferences, Soviet representatives had made known their interest in Turkish concession of Soviet naval bases on the straits. This, as well as Stalin's demands for the restitution of the Turkish provinces of Kars, Artvin and Ardahan to the Soviet Union (which were lost by Turkey in the Russo–Turkish War of 1877–1878, but were regained with the Treaty of Kars in 1921), were considerations in Turkey's decision to abandon neutrality in foreign affairs. Turkey declared war against Germany in February 1945, but did not engage in offensive actions. Turkey also decided to join NATO in 1952.
In more recent years, the Turkish Straits have become particularly important for the oil industry. Russian oil, from ports such as Novorossyisk, is exported by tankers primarily to western Europe and the U.S. via the Bosporus and the Dardanelles straits. In 2011 Turkey planned a 50km channel through Silivri as a second waterway, reducing risk in the Bosphorus.
The waters of the strait are traversed by numerous ferries. Two suspension bridges cross the Bosporus. The first of these, the Bosporus Bridge, is 1,074 m (3,524 ft) long and was completed in 1973. The second, named Fatih Sultan Mehmet (Bosporus II) Bridge, is 1,090 m (3,576 ft) long, and was completed in 1988 about 5 km (3 mi) north of the first bridge. The Bosporus Bridge forms part of the O1 Motorway, while the Fatih Sultan Mehmet Bridge forms part of the Trans-European Motorway.
Construction of a third suspension bridge, the Yavuz Sultan Selim Bridge, began on May 29, 2013; opening is planned for May 29, 2015. The bridge will be built near the northern end of the Bosporus, between the villages of Garipçe on the European side and Poyrazköy on the Asian side. It will be part of the "Northern Marmara Motorway", which will be further integrated with the existing Black Sea Coastal Highway, and will allow transit traffic to bypass city traffic,
The Marmaray project, featuring a 13.7 km (8.5 mi) long undersea railway tunnel, opened on 29 October 2013. Approximately 1,400 m (4,593 ft) of the tunnel runs under the strait, at a depth of about 55 m (180 ft).
An undersea water supply tunnel with a length of 5,551 m (18,212 ft), named the Bosporus Water Tunnel, was constructed in 2012 to transfer water from the Melen Creek in Düzce Province (to the east of the Bosporus strait, in northwestern Anatolia) to the European side of Istanbul, a distance of 185 km (115 mi).
The phrase "swim the Bosporus" or "crossing the Bosporus" is used to indicate religious conversion to the Eastern Orthodox Church.
In 2010, a team of scientists led by the University of Leeds used a robotic "yellow submarine" to observe detailed flows within an "undersea river" for the first time. Submarine channels are similar to land rivers, but they are formed by density currents—underwater flow mixtures of sand, mud and water that are denser than sea water and so sink and flow along the bottom. These channels are the main transport pathway for sediments to the deep sea where they form sedimentary deposits. These deposits ultimately hold not only untapped reserves of gas and oil, they also house important secrets—from clues on past climate change to the ways in which mountains were formed.
The team studied the detailed flow within these channels and findings included:
The channel complex and the density flow provide the ideal natural laboratory for investigating and detailing the structure of the flow field through the channel. Our initial findings show that the flow in these channels is quite different to the flow in river channels on land. Specifically, as flow moves around a bend it spirals in the opposite direction in the deep sea compared to the spiral to that found in river channels on land. This is important in understanding the sedimentology and layers of sediment deposited by these systems.
The central tenet of the Black Sea Deluge theory is that as the ocean rose 238 ft at the end of the last Ice Age when the massive ice sheets melted, the sealed Bosporus was overtopped in a spectacular flood that increased the then fresh water Black Sea Lake 50%, and drove people from the shores for many months. This was proven by undersea explorer Robert Ballard, who discovered settlements along the old shoreline; scientists dated the Flood to 7500 BP or 5500 BC from fresh-salt water microflora. The peoples driven out by the constantly rising water, which must have been terrifying and inexplicable, spread to all corners of the Western world carrying the story of the Great Flood, how it probably entered most religions. As the waters surged, they scoured a network of sea-floor channels less resistant to denser suspended solids in liquid, which remains a very active layer today.
It is one of the possibilities for the biblical event of Noah's flood. The first images of these submarine channels were obtained in 1999, showing them to be of great size in the frame of a NATO SACLANT Undersea Research project using jointly the NATO RV Alliance, and the Turkish Navy survey ship Çubuklu. In 2002, a survey was carried out on board the Ifremer RV Le Suroit for BlaSON project (Lericolais, et al., 2003) completed the multibeam mapping of this underwater channel fan-delta. A complete map was published in 2009  using these previous results with high quality mapping obtained in 2006 (by researchers at Memorial University, Newfoundland, Canada who are project partners in this study.)
The team will use the data obtained to create innovative computer simulations that can be used to model how sediment flows through these channels. The models the team will produce will have broad applications, including inputting into the design of seafloor engineering by oil and gas companies.
The project was led by Dr. Jeff Peakall and Dr. Daniel Parsons at the University of Leeds, in collaboration with the University of Southampton, Memorial University (Newfoundland, Canada), and the Institute of Marine Sciences (Izmir, Turkey). The survey was run and coordinated from the Institute of Marine Sciences research ship, the R/V Koca Piri Reis.
The researchers estimate that the river—known as a submarine channel—would be the sixth largest river in the world if it were on land based on the amount of water flowing through it.
The Bosphorus has 620 waterfront houses (yalı) built during the Ottoman period along the strait's European and Asian shorelines. Ottoman palaces such as the Topkapı Palace, Dolmabahçe Palace, Yıldız Palace, Çırağan Palace, Feriye Palaces, Beylerbeyi Palace, Küçüksu Palace, Ihlamur Palace, Hatice Sultan Palace, Adile Sultan Palace and Khedive Palace are within its view. Buildings and landmarks within view include the Hagia Sophia, Hagia Irene, Sultan Ahmed Mosque, Yeni Mosque, Kılıç Ali Pasha Mosque, Nusretiye Mosque, Dolmabahçe Mosque, Ortaköy Mosque, Üsküdar Mihrimah Sultan Mosque, Yeni Valide Mosque, Maiden's Tower, Galata Tower, Rumelian Castle, Anatolian Castle, Yoros Castle, Selimiye Barracks, Sakıp Sabancı Museum, Sadberk Hanım Museum, Istanbul Museum of Modern Art, Borusan Museum of Contemporary Art, Tophane-i Amire Museum, Mimar Sinan Fine Arts University, Galatasaray University, Boğaziçi University, Robert College, Kabataş High School, Kuleli Military High School.
Two points in Istanbul have most of the public ferries that traverse the strait: from Eminönü (ferries dock at the Boğaz İskelesi pier) on the historic peninsula of Istanbul to Anadolu Kavağı near the Black Sea, zigzagging and calling briefly multiple times at the Rumelian and Anatolian sides of the city. At central piers shorter, regular ride in one of the public ferries cross.
Private ferries operate between Üsküdar and Beşiktaş or Kabataş in the city. The few well-known geographic hazards are multiplied by ferry traffic across the strait, linking the European and Asian sides of the city, particularly for the largest ships.
The catamaran seabuses offer high-speed commuter services between the European and Asian shores of the Bosphorus, but they stop at fewer ports and piers in comparison to the public ferries. Both the public ferries and the seabuses also provide commuter services between the Bosphorus and the Prince Islands in the Sea of Marmara.
There are also tourist rides available in various places along the coasts of the Bosphorus. The prices vary according to the type of the ride, and some feature loud popular music for the duration of the trip.
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