The River That Sinks Into the Ground
On many summer days, part of the Danube does something strange. It disappears. At two spots in the Swabian Alb in southern Germany, the river sinks completely into underground channels. This is called the Donauversickerung, or Danube Sink.
The rock there is porous limestone, and the Rhine sits much lower than the Danube. So the water slips underground and heads for the Rhine instead. Most of it comes back up only 12 km (7.5 mi) south at the Aachtopf, the German spring with the highest flow. From there it feeds the Rhine.
This is a slow-motion takeover. The moving water keeps wearing away the limestone, and it is predicted that the upper Danube will one day vanish entirely in favour of the Rhine, a process called stream capture. It has happened before. Before the last ice age, Alpine water that now feeds the Rhine flowed east in a much bigger original Danube. Today's upper Danube is the small leftover, an underfit stream.
Ten Countries and Four Capitals
The Danube is Europe's second-longest river, after the Volga. It starts in the town of Donaueschingen in Germany's Black Forest, where two small rivers, the Brigach and the Breg, meet. From there it flows southeast for about 2,850 km (1,770 mi) until it empties into the Black Sea.
On the way it passes through or borders ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine. It also runs through four national capitals: Vienna, Bratislava, Budapest and Belgrade. That is more capitals than any other river in the world.
The land that drains into the river, its drainage basin, reaches into nine more countries, from Switzerland to Albania. About 83 million people live there. The highest point of the whole basin is the summit of Piz Bernina, on the Italy-Switzerland border, at 4,049 m (13,284 ft). Down near the other end, Galați in Romania is the largest port on the river.
A Water Highway Across Europe
Since ancient times, the Danube has been one of Europe's main trade routes. The Romans used it as a border and also as a road for moving troops and supplies. Today about 2,415 km (1,501 mi) of it is navigable. Ocean ships can sail up to Brăila in Romania, and river ships can reach Kelheim in Bavaria.
In 1992 Germany finished the Rhine-Main-Danube Canal. Since then, boats can travel 3,500 km (2,200 mi) from Rotterdam on the North Sea to Sulina on the Black Sea. Another shortcut, the Danube-Black Sea Canal, finished in 1984, cuts 400 km (250 mi) off the trip to the sea.
River life was not always gentle. In the 19th century, The Times of London warned that the Danube was swept every year by ice that could lift a large ship out of the water or cut her in two as if she were a carrot. Locals even built a flat-bottomed boat, the Zille, just for this river.
Europe's Best-Kept Delta
At its end, the Danube splits into three main channels and fans out into the Danube Delta, mostly in Romania with a northern part in Ukraine. It is the largest river delta in the European Union and the best-preserved one in Europe. It became a UNESCO World Heritage Site in 1991.
The delta is a giant rest stop for birds. Its wetlands host huge flocks of migratory birds from more than 300 species, including the endangered pygmy cormorant. Its lakes and marshes support 45 freshwater fish species.
The rest of the river has a big fish story too. The Danube was once home to six kinds of sturgeon, including the beluga, and people caught them for meat and caviar from the 5th century BC. Today all six are threatened. The huchen, one of the largest kinds of salmon, is endemic to the Danube basin, though humans have since carried it to other places.
Waltzes, Bike Trails and a Mammoth
The river gave its name to one of the world's best-known tunes. Austrian composer Johann Strauss wrote The Blue Danube waltz, a piece famous across the globe and widely used as a lullaby. The Danube also features in Bulgaria's national anthem as a symbol of the country's natural beauty. In Lithuanian folk songs, it shows up more often than Lithuania's own longest river, the Neman.
You can follow it on two wheels, too. The Danube Bike Trail runs from the river's source all the way to the Black Sea, in four sections. Its German and Austrian parts make it one of the 10 most popular bike trails in Germany.
Sometimes the Danube even gives up its secrets. In July and August 2026, a long heatwave and drought pushed water levels to record lows. Near Ryahovo in northern Bulgaria, the shrinking river revealed the remains of a young mammoth, including a jawbone, two tusks and rib bones.
