The Blautopf (German for Blue Pot; "blau" means blue, "Topf" means pot) is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Jura's southern edge, in Southern Germany. It is located in the city of Blaubeuren, approximately 16 km (9.9 mi) west of Ulm. It forms the drain for the Blau cave system and feeds the river Blau, which after 14.5 km (9.0 mi), flows into the river Danube in the city of Ulm. Because of its high water pressure, the spring has developed a funnel-like shape, which at its deepest point has a depth of 21 metres (69 ft). The water's peculiarly blue color, varying in intensity due to weather and flow, is the result of physical properties of the nanoscale limestone densely distributed in the water. The particles are so small that the so-called Rayleigh scattering of light takes place, preferentially scattering the blue color in the visible light. Similar effect is observed at the Blue Lagoon in Reykjavik, Iceland, where the blue color originates from nanoscale silica particles.
Numerous legends and folk tales refer to the Blautopf. Its characteristic colour was explained by the account that every day someone would pour a vat of ink into the Blautopf. Another myth stated that every time someone tried to measure the Blautopf's depth with a leaden sounding line, a water nix stole the sounding line. Therefore, it was not possible to determine the depth of the Blautopf. Because of this tale, there is a rock called Klötzle Blei ("little block of lead" in the local dialect) in the vicinity of the Blautopf.
The Blautopf is a spring in a Karst environment. One characteristic of a Karst environment is that water, which drains quickly through the limestone in one area, surfaces in another. Karst environments only have subterranean drainage, and there are no bodies of water above ground. Therefore, the size of the Blautopf depends greatly on the level of rainfall, though it never entirely dries out. The Blautopf is the second largest spring in Germany, after the Aachtopf. Over millennia, subterranean water has created a huge system of caves in the area. Prominent examples are the Blauhöhle (Blau-cave), discovered by Jochen Hasenmayer in 1985,[1] and the Apokalypse (Apocalypse), discovered on 23 September 2006 by Jochen Malmann and Andreas Kücha, members of the Arbeitsgemeinschaft Blautopf, a club dedicated to the exploration of the Blautopf's cave system.[2] While the Blauhöhle is completely filled with water for a length of about 1500 metres (approximately 4935 ft), the Apokalypse is dry; because of its dimensions-170 metres long, 50 metres wide, 50 metres high-it is a special feature of the region.