Danube
Classified as an international waterway, it originates in the town of Donaueschingen, in the Black Forest of Germany, at the confluence of the rivers Brigach and Breg. The Danube then flows southeast for about , passing through four capital cities (Vienna, Bratislava, Budapest and Belgrade) before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
Danube Sinkhole
The sinking Danube water disappears into a karst water system of the "well-stratified limestone formation" (the "ox2" layer) of the White Jura and appears again in a horizontal limestone layer (the "ki4" layer), approximately twelve kilometers away at Aachtopf. It then flows as Radolfzeller Aach into Lake Constance at Radolfzell. Thus, a part of the Danube water also flows into the Rhine. This geographical situation is a striking feature of the large European Watershed, which separates the catchment areas of the North Sea and the Black Sea.