The Black Sea is part of the North Atlantic Ocean and lies north of Anatolia, between the Balkans and the Caucasus. It is connected to the Sea of Azov via the Kerch Strait and the Sea of Marmara through the Bosporus Strait, which connects it to the Atlantic. It covers an area of around 436,000 km2. The Black Sea is supplied by the Danube, the Dnieper and the Don. The Black Sea used to be a large freshwater lake, but when the sea levels rose after the ice age, it became connected to the Mediterranean Sea, turning it into a sea.