![The Dardanelles and Bosporus Passages | Port Economics, Management and Policy](https://i0.wp.com/porteconomicsmanagement.org/wp-content/uploads/Map-Bosporus.png?resize=900%2C649&ssl=1)
The dividing line between Europe and Asia, the link-up between the Mediterranean and the Back Sea, the Bosporus is an ancient valley along a fractured tectonic line. From the Mediterranean Ocean, it runs through the Dardanelles passage to reach the Sea of Marmara, a small interior sea. Then, the narrow Bosporus passage (1 km at its narrowest point) goes through Istanbul to reach the Black Sea.