The Aachtopf is Germany's biggest natural spring, producing an average of 8,500 litres per second. Production varies seasonally and in response to the weather, but the spring never runs dry. The Aachtopf is a karst spring which is located south of the western end of the Swabian Jura, near the town Aach.
The name Aachtopf is compounded from Aach (meaning water in Old High German); the name of the river created by the spring is in full the Radolfzeller Aach. Topf can be translated as bowl and is commonly used for round, bowl-shaped springs. The Radolfzeller Aach flows southward into Lake Constance, which empties into the Rhine.
The spring marks the southern end of a cave system which transports water from the western end of the Swabian Jura. Most of the water is derived from the River Danube and is obtained where the latter river disappears underground at the Donauversickerung (Danube Sinkhole) near Immendingen and Fridingen. Strangely, 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 continental divide. 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. The first exploration was made by Jochen Hasenmayer, a famous German cave diver. Unfortunately, a collapse blocks the cave after a few hundred metres. The cave may continue for several kilometres beyond the other side of the blockage. A local caving club was formed to find this "lost cave segment" by digging a shaft behind the collapse. At present, a continuation of the cave has not been found.
The Aachtopf is a popular romantic weekend destination. It is not possible to see the cave because it is underwater and cave diving is extremely dangerous.