The Bosporus is a natural strait connecting the Sea of Marmara with the Black Sea in northern Turkey. It is the narrowest strait used for international navigation, and an internationally important waterway which, together with the Dardanelles, forms the Turkish Strait which connects the Black Sea basin to the Mediterranean basin and the Atlantic Ocean. The strait comprises the division between European Turkey and Asian Turkey. It is heavily settled on both shores, straddled by the metropolitan area of Istanbul. Two suspension bridges and a cable-stayed bridge cross the Bosporus, and three tunnels cross under it.