The Bosphorus

Few cities have a shipping lane for a main street. The Bosphorus runs straight through Istanbul, a city of more than 15 million, dividing it between Europe and Asia, and tankers and bulk carriers glide past Ottoman palaces, mosques, and waterfront teahouses while commuter ferries dodge between them. It is the narrowest strait used by international shipping, and with the Dardanelles it forms the Turkish Straits, the Black Sea's only route to the Mediterranean. Grain from Ukraine and Russia, oil from Novorossiysk, and the warships of every Black Sea navy have to pass this way, under rules written at Montreux in 1936 that still govern it today.

The strait, called İstanbul Boğazı in Turkish, runs about 31 kilometers from the Black Sea south to the Sea of Marmara, winding through wooded hills in a series of sharp bends. It is about 3.4 kilometers across at its widest and only some 700 meters at its narrowest, between Kandilli on the Asian shore and the Rumeli Fortress on the European side, with midstream depths from about 13 to 110 meters. A surface current carries brackish Black Sea water south while saltier Mediterranean water flows north beneath it. Three suspension bridges span it, opened in 1973, 1988, and 2016, and two tunnels run under it, the Marmaray rail link of 2013 and the Eurasia road tunnel of 2016. In 2024 about 41,000 ships made the passage, around 9,000 of them tankers.

Byzantium was founded on its western shore, and Constantinople and then Istanbul grew rich and powerful by controlling it. Russia's centuries-long push for warm-water access made the straits a recurring cause of war. The 1936 Montreux Convention guarantees merchant ships free passage in peacetime, with no transit tolls though Turkey charges service fees, and caps the number, size, and stay of warships from non-Black Sea states. In wartime, if Turkey is neutral, belligerent warships may not pass except to return to base. Turkey invoked that clause days after Russia's full-scale invasion of Ukraine in February 2022, and in 2024 it also declined passage to British minehunters bound for Ukraine. Commercial traffic continues, though drone strikes on tankers in the Black Sea since late 2025 have kept insurers wary. A proposed parallel waterway, Kanal Istanbul, has not been built.

• Black Sea
  A vast, near-landlocked sea with a lifeless deep connects
• Mediterranean Sea
  The cradle sea of Western civilization, locked between three continents connects toward
• The Byzantine Empire
  Romes Greek-speaking eastern half, ruled from Constantinople for more than a thousand years controlled by
• The Ottoman Empire
  Six centuries of rule from Istanbul over the Balkans, Anatolia, the Arab world and North Africa controlled by
• Istanbul
  The city straddling Europe and Asia across the Bosphorus flows through
• Turkey
  A transcontinental bridge between Europe and Asia located in
• The Dardanelles
  The narrow Turkish strait between Europe and Asia that links the Sea of Marmara with the Aegean and the Mediterranean paired strait
• Black Sea
  A vast, near-landlocked sea with a lifeless deep sea exit of
• Russia
  The largest country on Earth, spanning Europe and Asia across eleven time zones wartime passage restrictions
• Ukraine
  Europe's breadbasket of black-earth steppe, the Dnieper, and the Black Sea wartime passage restrictions