The Danube Sinkhole ( German: Donauversinkung or Donauversickerung) is an incipient underground stream capture in the Upper Danube Nature Park. Between Immendingen and Möhringen and also near Fridingen ( Tuttlingen), the water of the Danube sinks into the riverbed in various places. The main sinkhole is next to a field named Brühl between Immendingen and Möhringen.

The term “sinking” is more accurate than “seeping”, because, instead of just distributing into the soil, the Danube’s water flows through underground caverns to the Aachtopf, where it emerges as the river Radolfzeller Aach.

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.

Hans Bayer, known by the pseudonym Thaddäus Troll, (18 March 1914 – 5 July 1980) was a German journalist and writer and one of the most prominent modern poets in the Swabian German dialect. In his later years he was also an active campaigner for libraries and for support, pension rights and fair publishing contracts for writers. He was born in Bad-Cannstatt, a suburb of Stuttgart and committed suicide there at the age of 66. The literary award, Thaddäus-Troll-Preis, is named in his honour. His family had a soap-making business in the town. After he finished his secondary education at the Johannes-Kepler-Gymnasium, he worked briefly as a volunteer at a newspaper in Cannstatt. He then studied German, art history, comparative literature, theater, and journalism at the universities of Tübingen, Munich, Halle, and Leipzig, receiving his doctorate in 1938 from Leipzig University.