Blautopf

thumb|right|Blautopf with the bright limestone sediments underwater thumb|Blautopf with hammer mill The Blautopf (, ) is a spring that is considered the source of the river Blau in the karst landscape on the Swabian Jura's southern edge. It is located in Blaubeuren, Alb-Donau-Kreis, Baden-Württemberg, Germany (approximately west of Ulm).

==Description== It forms the drain for the Blau cave system; the river Blau after flows into the river Danube in the city of Ulm. Because of its high water pressure, the spring has developed a funnel-like shape with a depth of 21 metres (69 ft). The water's peculiarly blue color, varying in intensity depending on weather and flow, is the result of physical properties of the nanoscale limestone particles densely distributed in the water. They cause Rayleigh scattering of light, preferentially scattering the blue color of the visible light. A similar effect is observed at the Blue Lagoon near Reykjavík, where the color originates from nanoscale silica particles.