Bosphorus Strait

The Bosphorus (Istanbul Strait) is the roughly 31 km Turkish waterway linking the Black Sea to the Sea of Marmara and onward through the Dardanelles to the Mediterranean. Transit by merchant ships is governed by the Montreux Convention of 1936, which guarantees free passage in peacetime while letting Turkey regulate warships and close the straits to belligerents in war. With a navigable width narrowing to about 700 m and sharp bends, it’s a chokepoint for Russian, Ukrainian, and Caspian oil and grain; Turkey adds traffic-separation and pilotage rules.