The Bosphorus, also known as the Strait of Istanbul, is a narrow, natural strait and an internationally significant waterway located in northwestern Turkey. It forms part of the continental boundary between Europe and Asia, and divides Turkey by separating Anatolia from Thrace.
It is the world's narrowest strait used for international navigation, the Bosporus connects the Black Sea with the Sea of Marmara, and, by extension via the Dardanelles, the Aegean and Mediterranean seas.
Most of the shores of the strait are heavily settled, straddled by the city of Istanbul's metropolitan population of 17 million inhabitants extending inland from both coasts.
According to en.wikipedia
