Forming the headwaters of the river Blau, the Blautopf arises from the Swabian Jura mountains of southern Germany. Meaning “Blue Pot” in German, the spring’s dazzling color lends itself easily to myth and fascination.

The Blautopf is actually one endpoint of a vast underground cave system in the area’s karst landscape. Over millennia, erosion has dug an intricate and ever-growing network of tunnels through the region’s soft limestone, creating a complex underground drainage system.

Any precipitation is quickly channeled into these caves, running down and through the natural conduits until the aggregating pressure pushes the water back above ground somewhere. The Blautopf is one such “somewhere,” a place where the water in the Blau cave system can drain out, forming the Blau river and eventually flowing into the Danube. Richly saturated with limestone nanoparticles by the time it returns to the surface, the water takes on its striking hue thanks to Rayleigh scattering, which is the same phenomenon that makes the sky blue.

Due to the high pressure of the water issuing from the bottom of the spring, the Blautopf has a steep funnel-like shape, measuring 21 meters (69 feet) at its deepest point. It is no surprise, then, that this strangely colored, remarkably deep pool has had a few legends associated with it over the centuries.

One such legend attributed the color of the spring to the fact that someone dumped a vat of ink in it every day; another claimed that the Blautopf’s depth could not be measured because any sounding line lowered into its waters would be stolen by a water nix (an often malicious Germanic water spirit).

The most famous legend associated with the Blautopf is that of “Die Schöne Lau,” a beautiful mermaid who was incapable of laughing and could only bear stillborn children. According to legend, her husband (the King of the Black Sea) responded to this situation by banishing her to the Blautopf until she managed to laugh five times, which would break the stillborn curse.

Today, the Blautopf is an important but extremely dangerous access point for the exploration for the majestic Blau cave system and other associated underground formations. The system can be entered 18 meters down via an enormous underwater cave called the Blauhöhle. This was once a popular site for cave divers, but access is now tightly restricted after several people met their doom in these treacherous watery chambers (cave diving is risky in general; diving the Blauhöhle particularly so). Probably best, then, to just enjoy the view from the shore, and not tangle with the malicious nixes lurking beneath the Blautopf’s surface.