Bosphorus

The Bosporus Strait or Bosphorus Strait is a natural strait and an internationally significant waterway located in Istanbul in northwestern Turkey. It forms part of the continental boundary between Asia and Europe, and divides Turkey by separating Anatolia from Thrace. It is the world's narrowest strait used for international navigation.

Most of the shores of the Bosporus Strait, except for the area to the north, are heavily settled, with the city of Istanbul's metropolitan population of 17 million inhabitants extending inland from both banks.

Source: Wikipedia