The Danube is Europe's second longest river after the Volga and the longest river in the European Union. It originates in Germany's Black Forest as the much smaller Brigach and Breg rivers, which join at the town of Donaueschingen. From this point, it is known as the Danube, and flows eastward for a distance of 1776 miles (2857 km), passing through several Central and Eastern European capitals before emptying into the Black Sea via the Danube Delta in Romania and Ukraine. Known to history as one of the long-standing frontiers of the Roman Empire, the river flows through—or forms a part of the borders of—ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine. In addition, the drainage basin includes parts of nine more countries: Italy, Poland, Switzerland, Czech Republic, Slovenia, Bosnia and Herzegovina, Montenegro, Republic of Macedonia, and Albania. The Danube's Basin drains 315,445.5 square miles (817,000 sq km), making it the most international river in the world. Considered Europe's lifeline, the basin is home to more than 83 million people and diverse and unique habitats supporting a multitude of species of animals and plants, some rare and endangered. Germany’s Black Forest, the Alps, and the Carpathian Mountains are a part of the Danube Basin. Not only does the Danube serve both the human and the natural world in very practical ways, it is in itself a work of art and source of inspiration. Johann Strauss composed the famous Blue Danube Waltz, while German poet Friedrich Holderlin called the Danube "a refreshing, melodious river, sometimes foaming with high spirits, at other times dreaming serenely."