Bosphorus is a natural strait connecting the Black Sea to the Sea of Marmara, thus being a very strategic waterway. It was a river in the valley during the Tertiary period of the continents, which was drowned by the sea at the end of this period. It’s length is 32 kilometers (20 miles) in the north to south direction, width varies between 730-3300 meters (800-3600 yards), and depth is between 30-120 meters (100-395 feet). Bosphorus strait separates the European part from the Asian part of Istanbul. The surface current flows always from north to south; however, a strong countercurrent under the surface creates swirls and eddies
