The Danube Sinkhole ( or ) 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 caverns to the Aachtopf, where it emerges as the river Radolfzeller Aach, a tributary of the Rhine. Hydrography 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. The water flows out through a variety of small to very small cracks and crevices; the karst in these places is apparently at an early stage of development. The cave system of Aachtopf, which is the underground river Danube is, however, probably already well developed; This can be inferred from the tight correlation in water temperatures of the Danube and the Aachtopf spring, which matches the behaviour of an underground river better than that of a branching system. History The first documented case of the Danube completely disappearing into the hole dates back to 1874. Since then, the number of days per year when this happens has risen sharply. Between 1884 and 1904, it happened, on average, 80 days per year. In 1922, it happened only on 29 days. In 1923, however, the number increased to 148. Between 1933 and 1937, the average was on 209; from 1938 to 1945, it was 270 days. The highest number so far, was in 1921, with 309 days. The relationship between the sinkhole and Aachtopf spring was proven in October 1877. Geologist Adolf Knop of the Karlsruhe Institute of Technology poured 10 kg of sodium fluorescein, 20 tons of salt, and 1200 kg of shale oil into the Danube on 9 October. After 60 hours, all three substances emerged at the Aachtopf. The water