The Bosporus Strait, also called the Bosphorus, is a natural waterway in Turkey. The city of Istanbul is located across this strait. The Bosporus connects the Black Sea to the Sea of Marmara and separates the continents of Asia and Europe. It also divides Turkey by separating Asia Minor from Thrace. The Bosporus is the narrowest strait in the world used for international ship travel.
Most of the land along the Bosporus, except for the northern area, is heavily populated. The city of Istanbul has a large population of 17 million people living near both sides of the strait.
The Bosporus Strait and the Dardanelles Strait, which is at the other end of the Sea of Marmara, are together called the Turkish Straits. Parts of the Bosporus shoreline in Istanbul are reinforced with concrete or rubble. Areas of the strait where sediment builds up are regularly cleared to keep the waterway open.
Name
The name of the strait comes from the Ancient Greek word Βόσπορος (Bósporos), which was later misunderstood as βοὸς πόρος, meaning "cattle strait" (like "Ox-ford"). This comes from the Greek words βοῦς (boûs), meaning "ox or cattle," and πόρος (poros), meaning "passage." The name refers to a Greek myth about Io, who was turned into a cow and forced to wander the Earth until she crossed the Bosporus. There, she met Prometheus, who told her she would be changed back into a human by Zeus and become the ancestor of Heracles (Hercules).
Io is said to have landed near Chrysopolis, now called Üsküdar, which was once named Bous, meaning "the Cow." The same place was also called Damalis, named after Damalis, the wife of an Athenian general, Chares. He built a monument there that included a large statue of a cow.
The English spelling "Bosphorus" with "-ph-" is not based on the ancient Greek name. Dictionaries prefer the spelling with "-p-" (Bosporus). However, the "-ph-" spelling appears in some medieval Latin and Greek texts, leading to variations in other languages, such as French (Bosphore), Spanish (Bósforo), Italian (Bosforo), and Russian (Босфор). In the 12th century, a Greek scholar named John Tzetzes called the strait "Damaliten Bosporon" after Damalis. He also noted that people sometimes called it "Prosphorion," the name of an ancient harbor in Constantinople. In English, "Bosphorus" is the preferred spelling.
Historically, the Bosporus was also called the "Strait of Constantinople" or the "Thracian Bosporus" to distinguish it from the "Cimmerian Bosporus" in Crimea. These names appear in the writings of the ancient historian Herodotus. He also used names like "Chalcedonian Bosporus" and "Mysian Bosporus" to describe the strait.
Over time, the word "Bosporus" became a general term for "a strait" in Greek. It was also used to refer to the Hellespont, a nearby waterway, in the works of ancient Greek playwrights like Aeschylus and Sophocles.
The area known as Boğaziçi includes parts of Istanbul and nearby regions that have a view of the Bosporus. It is especially associated with the northern areas of Istanbul, starting with the district of Beşiktaş on the European side of the strait. From south to north, the European side includes Beşiktaş, Ortaköy, Arnavutköy, Bebek, Rumelihisarı, Baltalimanı, Emirgan, İstinye, Yeniköy, Tarabya, Kireçburnu, Büyükdere, and Sarıyer. The Asian side includes Üsküdar, Beylerbeyi, Çengelköy, Vaniköy, Kandilli, Anadolu Hisarı, Kanlıca, and Beykoz.
Geography
The Bosporus is a waterway that connects the Black Sea to the Sea of Marmara and then to the Aegean and Mediterranean seas through the Dardanelles. It also links various seas in the Eastern Mediterranean, the Balkans, the Near East, and Western Eurasia. This connection allows ships to travel from the Black Sea to the Mediterranean Sea and the Atlantic Ocean through Gibraltar, and to the Indian Ocean via the Suez Canal. Because of this, the Bosporus is a key international waterway, especially for transporting goods from Russia.
There is a small island in the Bosporus near Kuruçeşme. Now called Galatasaray Island, it was given to the Armenian architect Sarkis Balyan by Sultan Abdülhamid II in 1880. Balyan built a house there, which was later demolished. The island became a walled garden and then a water sports center before being given to the Galatasaray Sports Club, which explains its name. In the 2010s, the island was filled with nightclubs, but the government removed them in 2017. It reopened in 2022 exclusively for members of the Galatasaray club.
Geologists still debate when and how the Bosporus formed. One idea, called the Black Sea deluge hypothesis, suggests that the Bosporus was flooded around 5600 BC (later revised to 6800 BC) when rising Mediterranean and Sea of Marmara waters broke into the Black Sea, which was then a freshwater lake. However, many scientists believe the strait is older, even though it is relatively young on a geological timescale.
The Bosporus is defined by the line connecting two lighthouses: Rumeli Feneri and Anadolu Feneri in the north, and Ahırkapı Feneri and Kadıköy İnciburnu Feneri in the south. The strait is 31 kilometers long, with a width of 3,329 meters at the northern entrance and 2,826 meters at the southern entrance. Its widest point is 3,420 meters between Umuryeri and Büyükdere Limanı, and its narrowest is 700 meters between Kandilli Point and Aşiyan.
The depth of the Bosporus ranges from 13 to 110 meters, with an average of 65 meters. The deepest spot is between Kandilli and Bebek at 110 meters, while the shallowest areas are near Kadıköy İnciburnu (18 meters) and Aşiyan Point (13 meters). Water flows south at 16,000 cubic meters per second (freshwater at the surface) and north at 11,000 cubic meters per second (saltwater near the bottom). Researchers from the University of Leeds describe a "Black Sea undersea river."
The Golden Horn is an estuary near the main strait that once protected Constantinople from attacks and provided shelter for imperial navies. After the 19th century, it became a historic neighborhood in Istanbul.
Before the 20th century, scientists knew that the Black Sea and the Sea of Marmara flow into each other through a "density flow." In 2010, a continuous underwater channel was discovered along the Bosporus floor. If it were on land, this channel would be the sixth-largest river. Scientists used a robotic "yellow submarine" to study the channel, which is called a submarine channel. These channels are formed by dense mixtures of sand, mud, and water that flow along the ocean floor. They help transport sediments to the deep sea, where they form layers of sediment.
Studies of the channel showed that the flow in submarine channels differs from land rivers. For example, water spirals in the opposite direction around bends in the deep sea compared to land rivers. This helps scientists understand how sediments are deposited.
The Black Sea deluge hypothesis claims that rising sea levels after the last Ice Age caused a flood that increased the Black Sea’s size by 50%. This event, discovered by underwater explorer Robert Ballard, may have forced people to move, spreading stories of a major flood. The flood carved channels on the seafloor, which remain active today.
Images of these submarine channels were first taken in 1999 during a NATO project. In 2002, a survey completed mapping the channel using a ship called the Le Suroit. A full map was published in 2009 using data from earlier studies and a 2006 survey by researchers at Memorial University of Newfoundland.
The project was led by scientists at the University of Leeds, working with the University of Southampton, Memorial University of Newfoundland, and the Institute of Marine Sciences. The research was conducted from the R/V Koca Piri Reis.
Small fishing villages once lined the Bosporus, growing since Byzantine times and becoming more prominent in the 19th century. Before the early 20th century, these villages were only accessible by boat (called caiques) because there were no roads. Today, they are suburbs of Istanbul but still have names ending in "-köy" (meaning "village"), such as Ortaköy, Yeniköy, Arnavutköy, Çengelköy, and Vaniköy. These villages were known for agriculture, like strawberry-growing in Arnavutköy and sweet cucumbers in Çengelköy.
Human history
The Bosporus is the only passage between the Black Sea and the Mediterranean. It has always been important for trade and military reasons, and it remains strategically important today. Many countries, including Russia and Ukraine, use the Bosporus as a major sea route. Control over it has been a goal in several conflicts, such as the Russo-Turkish War (1877–78) and the Allied attack on the Dardanelles during the 1915 Battle of Gallipoli in World War I. In 2022, during the Russian invasion of Ukraine, the Bosporus became important again as a route for grain to reach the world.
The Bosporus has been strategically important for thousands of years. In the 5th century BC, the Greek city-state of Athens relied on grain from Black Sea ports like Scythia. Athens formed alliances with cities that controlled the Bosporus, such as the Megarian colony of Byzantium.
In an effort to defeat the Scythian horsemen in the north of the Black Sea, the Persian King Darius I the Great (r. 522 BC–486 BC) crossed the Bosporus. His army used a large bridge made of connected boats to cross the strait, linking Asia to Europe. Later, Xerxes I built a similar boat bridge across the Dardanelles (Hellespont) in 480 BC during his invasion of Greece.
The Byzantines called the Bosporus "Stenon" and used specific names for places in the area. The strategic importance of the Bosporus influenced the Roman Emperor Constantine the Great to build his new capital, Constantinople, there in AD 330. This city later became the capital of the Eastern Roman Empire.
On 29 May 1453, the Ottoman Empire conquered Constantinople after a long campaign. The Ottomans built forts on both sides of the Bosporus—Anadoluhisarı (Anatolian Castle, 1393) and Rumelihisarı (European Castle, 1451)—to prepare for battle and control the strait. The 53-day campaign that ended with Ottoman victory marked a major turning point in history. This event, along with Christopher Columbus’s first voyage to the Americas in 1492, is often seen as ending the Middle Ages and starting the Renaissance and the Age of Discovery.
The conquest also ended the Byzantine Empire, the last part of the Roman Empire, and placed the Bosporus under Ottoman control. The Ottomans made Constantinople their capital and used it as a base to expand their empire.
Between the 16th and 18th centuries, the Ottoman Empire used the Bosporus to control the Black Sea, which they called an "Ottoman lake." Russian warships were not allowed to enter this area.
Several treaties later governed access to the Bosporus. The Treaty of Hünkâr İskelesi (8 July 1833) allowed Russia to close the Bosporus and Dardanelles to other nations’ warships. The London Straits Convention (13 July 1841) restored the Ottoman Empire’s rule, closing the straits to all warships except those of the Sultan’s allies during wartime. This helped British naval power but limited Russian access to the Mediterranean.
In the 19th century, foreign embassies in Constantinople moved to summer homes along the Bosporus during hot summers. These embassies were located on the European shore at places like Yeniköy, Tarabya, and Büyükdere. Some buildings still exist today, though the British Summer Embassy burned down in 1911, and the Italian Summer Embassy is in poor condition.
After World War I, the 1920 Treaty of Sèvres made the Bosporus an international territory under the League of Nations. This was changed by the 1923 Treaty of Lausanne, which gave Turkey control of the straits but allowed free passage for all ships. Turkey later rejected these terms and retook control of the area. The Montreux Convention (1936) still governs the straits today, allowing free shipping but letting Turkey restrict non-Black Sea nations’ naval traffic.
Turkey remained neutral in World War II until February 1945, closing the straits to enemy warships. Some German ships were allowed to pass. Soviet leaders wanted naval bases on the straits and to reclaim lost territories, which influenced Turkey’s decision to end its neutrality. Turkey declared war on Germany in 1945 but did not attack.
Turkey joined NATO in 1952, making the Bosporus even more important for trade and military purposes.
In the early 21st century, the Bosporus became vital for the oil industry. Russian oil from ports like Novorossiysk was exported to the U.S. through the Bosporus and Dardanelles.
In 2011, Turkey proposed building a 50 km (31 mi) canal near the Bosporus to reduce risks from oil tankers. The idea was controversial, and by 2022, no work had started on the canal, though a route was planned.
In mythology
The Bosphorus is named after a story from Greek mythology about a woman named Io. She was changed into a cow and chased by a gadfly, which forced her to wander the Earth until she reached the strait. There, she met Prometheus, a Titan, who told her that Zeus would one day return her to human form and make her the ancestor of Heracles, the greatest hero in Greek mythology.
In ancient Greek stories, it was said that huge floating rocks called the Symplegades, or Clashing Rocks, once blocked both sides of the Bosphorus. These rocks crushed any ship that tried to pass through the strait. The danger was finally stopped by Jason, a hero of the Argonauts, who successfully sailed between the rocks without harm. After this, the rocks stopped moving, allowing Greek ships to travel safely into the Black Sea.
Crossings
The two sides of the Bosphorus Strait are connected by four highways, which include three bridges and one undersea road tunnel, as well as one railway tunnel. There are also many maritime routes. A new railway tunnel for Istanbul’s Metro system is planned to open in 2028.
The Bosphorus is crossed daily by many ferries that carry passengers and vehicles, as well as recreational and fishing boats, including small dinghies and large yachts owned by public and private groups.
The strait also carries a large amount of international commercial shipping, such as freighters and tankers. Between its northern limits at Rumeli Feneri and Anadolu Feneri and its southern limits at Ahırkapı Feneri and Kadıköy İnciburnu Feneri, there are several dangerous areas for large ships. These areas require sharp turns and involve visual obstructions. For example, the section between Kandilli Point and Aşiyan requires a 45-degree change in direction, where currents can reach 7 to 8 knots (3.6 to 4.1 meters per second). Near Yeniköy, a course change of 80 degrees is needed. At both Kandilli and Yeniköy, sight lines are completely blocked before and during these turns, making it hard for ships coming from opposite directions to see around the bends. These challenges are made worse by the heavy ferry traffic that connects the European and Asian sides of Istanbul. All the dangers and obstacles typical of narrow waterways are present in this important sea route.
In 2011, the Turkish government began discussing the creation of a man-made canal about 80 kilometers (50 miles) long that would run north-south through the western edges of Istanbul Province. This canal would serve as a second route from the Black Sea to the Marmara Sea, reducing the risk of shipping accidents in the Bosphorus. The controversial Kanal İstanbul project is still being debated.
In 2022, Turkey increased the fees charged to freight ships by 500%, raising them to $4 per ton. This was the first change in fees since 1983.
The Bosphorus is fairly deep, with no strict limits on the size of ships. However, ships longer than 150 meters or deeper than 10 meters must book their passage in advance. Ships longer than 300 meters must follow a special approval process. There is also a limit of 57 meters for the height of ships above water.
Two suspension bridges and one cable-stayed bridge cross the Bosphorus. The first, the 1,074-meter (3,524-foot) long 15th July Martyrs Bridge, was completed in 1973 and was originally called the Bosporus Bridge. The second, the Fatih Sultan Mehmet (Bosporus II) Bridge, is 1,090 meters (3,576 feet) long and was completed in 1988, about 5 kilometers (3 miles) north of the first bridge. The 15th July Martyrs Bridge is part of the O1 Motorway, while the Fatih Sultan Mehmet Bridge is part of the Trans-European Motorway. The third and most recent bridge, the Yavuz Sultan Selim Bridge, is 2,164 meters (7,100 feet) long and was completed in 2016. It is located near the northern end of the Bosphorus, between Garipçe on the European side and Poyrazköy on the Asian side, and is part of the "Northern Marmara Motorway," connecting to the Black Sea Coastal Highway and allowing traffic to bypass city congestion.
The Marmaray project includes an undersea railway tunnel 13.7 kilometers (8.5 miles) long, which opened on October 29, 2013. About 1,400 meters (4,593 feet) of the tunnel runs under the Bosphorus at a depth of approximately 55 meters (180 feet).
An undersea water supply tunnel, called the Bosporus Water Tunnel, was built in 2012. It is 5,551 meters (18,212 feet) long and carries water from the Melen Creek in Düzce Province to the European side of Istanbul, a distance of 185 kilometers (115 miles).
The Eurasia Tunnel is a 5.4-kilometer (3.4-mile) undersea highway tunnel designed for vehicles between Kazlıçeşme on the European side and Göztepe on the Asian side. Construction began in February 2011, and the tunnel opened on December 20, 2016.
Up to four submarine fiber optic lines (such as MedNautilus and others) reach Istanbul, coming from the Mediterranean through the Dardanelles.
The Great Istanbul Tunnel is planned to open in 2028.
Strategic importance
The Bosporus is the only route for Bulgaria, Georgia, Romania, the southwestern part of Russia, and Ukraine to reach the Mediterranean Sea and other seas. Because of this, control over the straits is an important matter for these countries and Turkey, the country through which the Bosporus flows.
Turkey does not charge fees for ships passing through the strait. Turkey's military has a wide range of powers based on the Montreux Convention. In 2021, the Bosporus Command is located on the shores of the Bosporus, and military ships linked to the Command are anchored in the Bosporus waters.
Istanbul is located on a peninsula where the Black Sea, Bosporus, and Marmara Sea meet. Historically, it has been one of the most protected and difficult to conquer cities from Roman times to the Ottoman Empire. Divided by the Bosporus, Istanbul is one of the few cities in the world that is in two continents.
Sightseeing
The Bosporus has 620 waterfront homes called yalı built during the Ottoman period along the strait’s European and Asian shorelines. Ottoman palaces such as Topkapı Palace, Dolmabahçe Palace, Yıldız Palace, Çırağan Palace, Feriye Palaces, Beylerbeyi Palace, Küçüksu Palace, Ihlamur Palace, Hatice Sultan Palace, and Adile Sultan Palace are located on or near its shores. Other buildings and landmarks include the Kılıç Egyptian Consulate at Bebek, Bebek Mosque, Boğaziçi University, Robert College, Rumeli Castle, Borusan Museum of Contemporary Art, Sakıp Sabancı Museum, Sadberk Hanım Museum, Üsküdar Mihrimah Sultan Mosque, Şemsipaşa Mosque, Maiden’s Tower, Beylerbeyi Mosque, Anadolu Castle, Kuleli Military High School, Adile Sultan Palace, Küçüksu Pavilion, Khedive’s Villa, Beykoz Mecidiye Pavilion, and Yoros Castle.
Most public ferries that cross the strait begin at Eminönü on Istanbul’s historic peninsula and travel as far as Anadolu Kavağı near the Black Sea. These ferries briefly stop at points along both the European and Asian shores. Private ferries also start at Eminönü but travel only as far as the first two Bosporus bridges. Other ferries from Eminönü go to Rumeli Kavağı, stopping only on the European shore, while ferries from Üsküdar travel to Anadolu Kavağı, stopping only on the Asian shore. Public ferries frequently run from Eminönü, Karaköy, Beşiktaş, Kadıköy, and Üsküdar, offering short trips across the Bosporus throughout the day.
Catamaran sea buses provide fast travel between the European and Asian shores of the Bosporus but stop at fewer places compared to public ferries. Both ferries and sea buses also connect the Bosporus to the Prince Islands in the Sea of Marmara.
Tourist cruises operate from various points along the Bosporus, including Ortaköy. Prices vary, and some cruises include music and dining.
Architecture
The many yalı (waterside mansions) built along the Bosporus during the Ottoman period have been closely associated with the strait. Some of these mansions still look like they did when they were first built and are among the most valuable properties in Turkey. However, many have been damaged over time by weather, age, and fires. The oldest yalı on the European side is the Şerifler Yalı in Emirgan, built in the 18th century. It once belonged to the Şerifs, who were rulers of Mecca. This mansion remains in good condition. In contrast, the oldest yalı on the Asian side is the Köprülü Amcazade Hüseyin Paşa Yalı in Anadolu Hisarı, built in 1698. Only the middle part of this mansion still exists, and it has been protected by barriers since 2009 while plans for its restoration are made.
Most yalı were originally built right next to the water, with private docks where boats (called caiques) could be stored. On the Anatolian side, some yalı still sit close to the water. However, on the European side, many now stand behind a road built on land that was once part of the sea.
Original yalı usually had two main parts: the selamlık, which was the public area used by men, and the haremlik, which was the private area for women and family members. These homes were luxurious and some included private hamams (Turkish baths).
In the late 1800s and early 1900s, members of the Egyptian royal family enjoyed spending summers along the Bosporus. They built many fine homes on or near the shore. Examples include the building in Bebek that now serves as the Egyptian Consulate and the Khedive's Villa (Hıdiv Kasrı), located on a hill above Çubuklu.