The Black Sea is bordered by Ukraine, Romania, Bulgaria, Turkey, Georgia, and Russia. It has a positive water balance; that is, a net outflow of water 300 km3 (72 cu mi) per year through the Bosphorus and the Dardanelles into the Aegean Sea. There is a two-way hydrological exchange: the more saline and therefore denser, but warmer, Mediterranean water flows into the Black Sea under its less saline outflow. This creates a significant anoxic layer well below the surface waters. The Black Sea drains into the Mediterranean Sea, via the Aegean Sea and various straits, and is navigable to the Atlantic Ocean. The Bosphorus Strait connects it to the Sea of Marmara, and the Strait of the Dardanelles connects that sea to the Aegean Sea region of the Mediterranean. The Black Sea is also connected, to the north, to the Sea of Azov by the Kerch Strait.
The water level has varied significantly over geological time. Due to these variations in the water level in the basin, the surrounding shelf and associated aprons have sometimes been dry land. At certain critical water levels, connections with surrounding water bodies can become established. It is through the most active of these connective routes, the Turkish Straits, that the Black Sea joins the world ocean. During geological periods when this hydrological link wasn't present, the Black Sea was an endorheic basin, operating independently of the global ocean system (similar to the Caspian Sea nowadays). Currently, the Black Sea water level is relatively high; thus, water is being exchanged with the Mediterranean. The Turkish Straits connect the Black Sea with the Aegean Sea and comprise the Bosphorus, the Sea of Marmara and the Dardanelles. The Black Sea undersea river is a current of particularly saline water flowing through the Bosphorus Strait and along the seabed of the Black Sea. The discovery of the river announced on August 1, 2010, was made by scientists at the University of Leeds and is the first of its kind in the world. The undersea river stems from salty water spilling through the Bosphorus Strait from the Mediterranean Sea into the Black Sea, where the water has a lower salt content.