Aachtopf

The name Aachtopf is compounded from Aach (meaning "water" in Old High German); Topf can be translated as "bowl" and is commonly used for round, bowl-shaped springs. ==Geography==

The Aachtopf is a karst spring, south of the western end of the Swabian Jura near the town of Aach. The spring is the source of the river Radolfzeller Aach, which flows southward into Lake Constance, and empties into the Rhine. Origin The spring marks the southern end of a cave system that transports water from the western end of the Swabian Jura. It produces an average of 8,500 litres per second. Production varies seasonally and in response to the weather, but the spring never runs dry. Most of the water is derived from the Danube where it disappears underground at the Donauversickerung (Danube Sinkhole) near Immendingen and Fridingen. The Rhine's steeper gradient on its much shorter route to the North Sea creates stronger headward erosion than that of the much older River Danube (see the Danube upper river geology). This stream capture of the upper Danube and its surface tributaries is expected to cause the long-term disappearance of the Danube drainage entirely in favour of the Rhine. The Danube flows eastwards into the Black Sea, whereas the Rhine flows northwards to the North Sea. Therefore, the water of the Aach flows under the European watershed. This is a relatively common feature of karst stream captures. The karst spring is connected to a huge cave, which runs northwards. The cave is completely water-filled and can only be explored by cave divers. ==History of exploration==

In 1719, F. W. Bräuninger published his hypothesis that the water of the spring originated from the Danube. On 9 October 1877, the geologist Adolf Knop from Karlsruhe Technical College laced the water in the Danube basin with 10 kg sodium fluorescein, 20 zentner (1000 kg) of salt and 1200 kg of shale oil. After 60 hours he was able to detect all three substances in the spring, as evidenced by "splendidly green, salt water with a distinct creosote taste". In 1990, a local caving club was formed to find the lost cave segment towards the Danube basin by digging a shaft behind the collapse at the bottom of a large sink hole. In 1995, members discovered a cavern at deep, which they dubbed the Grey Hall, The group has published maps of the "Donauhöhle". As of 2020, exploration is ongoing and the continuation of the cave has not been discovered. ==Fauna==

In 2015, the only known cavefish in Europe, a Barbatula loach, was discovered in the Danube–Aachtopf system. ==References==