The Danube is the second-longest river in Europe, after the Volga. What makes the Danube truly unique is that it flows through or along the borders of 10 different countries—more than any other river in the world. Originating in the Black Forest of Germany, it flows 2,850 kilometers southeast before emptying into the Black Sea through the Danube Delta. It passes through four capital cities: Vienna, Bratislava, Budapest, and Belgrade. For centuries, it served as a frontier of the Roman Empire and remains a vital transport corridor for the European Union today. The Danube Delta, shared by Romania and Ukraine, is a UNESCO World Heritage site and the largest and best-preserved of European deltas. It is a critical habitat for migratory birds and home to over 300 species of avian life, making it a paradise for conservationists.