The Danube River is Europe's second-longest river, after the Volga. It is located in Central and Eastern Europe. The Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries, more than any other river in the world. Originating in Germany, the Danube flows southeast for 2,850 km, passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine before draining into the Black Sea. Since ancient times, the Danube has become a traditional trade route in Europe. Today 2,415 km of its total length is navigable. The river is also an important source of hydropower and drinking water.