The Blue Pot
The Blautopf (German for Blue Pot) is a natural 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 and it forms the drain for the Blau cave system and feeds the river Blau. 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, and 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.