The Danube is Europe's second-longest river, after the Volga. It originates in the Black Forest of Germany and flows 2,850 kilometers southeast, passing through or bordering ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine. It empties into the Black Sea through the Danube Delta, a UNESCO World Heritage site and the best-preserved delta in Europe. Historically, the Danube was a long-standing frontier of the Roman Empire. It passes through more capital cities (Vienna, Bratislava, Budapest, and Belgrade) than any other river in the world. It is a vital international waterway, used for freight, hydroelectricity, and tourism, though it faces environmental challenges from pollution and invasive species.