I got this information about Blautopf from Wikipedia:

The Blautopf (German for Bowl of the Blau, "blau" means blue) is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Alb's southern edge, in Southern Germany. It is located in the city of Blaubeuren, approximately 16 km (10 miles) west of Ulm. It forms the drain for the Blau cave system and feeds the river Blau, which after 14.5 km (9 miles), 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 blue colour is the result of chemical properties of limestone densely distributed in the water.

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 the centuries, subterranean water has created a huge system of caves. Prominent examples are the Blauhöhle (Blau-cave), discovered by Jochen Hasenmeyer in 1985, and the Apokalypse (Apocalypse), discovered on 23 September, 2006 by Jochen Malmann and Andy Kücha, members of the Arbeitsgemeinschaft Blautopf, a club dedicated to the exploration of the Blautopf's cave system. 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.

The entry to the Blauhöhle lies at a depth of about 18 metres (approximately 60 ft). Therefore, access is restricted to experienced and well-trained divers. In the 1980s, city authorities were forced to prohibit diving in the Blautopf after several accidents, including some fatal ones. Permission to dive in the Blautopf has only been granted to a few organizations: among them, the Arbeitsgemeinschaft Blautopf, a group of scientific speleologists led by Jochen Hasenmayer, and rescue services. The most recent fatal accident occurred in 2003, killing a member of Hasenmayer's team.