The Bosphorus separates the European side of Istanbul from its Asian side.
The Bosphorus is a narrow natural strait connecting the Black Sea with the Sea of Marmara. It runs through Istanbul, making the city famously span two continents. Its shores have supported settlements for thousands of years, and its position has given the waterway exceptional military and commercial importance.
The strait is approximately 31 kilometres long, though its width varies considerably. It forms part of the maritime route from the Black Sea to the Mediterranean through the Dardanelles and the Aegean Sea. A two-layer current system generally carries surface water southward while denser, saltier water flows northward below.
The Bosphorus is sometimes confused with the Dardanelles, which lies farther southwest and links the Sea of Marmara with the Aegean. Together, the two straits are known as the Turkish Straits. Several major bridges and a rail tunnel now connect Istanbul’s two sides, but shipping remains central to the waterway’s identity.