Alatsee, Mountain lake in Ostallgäu, Germany

Alatsee is a mountain lake in the Ostallgäu region that sits at an elevation of about 870 meters. It stretches several hundred meters in length and reaches depths of roughly 30 meters at its deepest point.

Near the lake a Celtic cult site has been documented, and gypsum was mined in the immediate surroundings until the end of the 19th century. During World War II, Luftwaffe technicians ran aerodynamic tests on and in the Alatsee with underwater models of the Focke-Wulf Ta 154; large iron rods and frames used for this remain in the water today. Towards the war's end, rumours held that Reichsbank gold stored at Neuschwanstein had been sunk in the lake, drawing treasure hunters in the 1950s and 1960s, who recovered mainly submerged weapons; no treasure was found, and diving has been banned since 1983.

Local lore holds that the lake is haunted by fabulous creatures and ghosts, and that careless hikers are pulled into the water or lured into crevices. Because fish and people have repeatedly vanished here, residents still consider it an ill-omened spot, though the deaths are most likely caused by escaping hydrogen sulphide. A tale blames the monks of the former St. Mang monastery, who supposedly caused a landslide that sent trees into the water.

The lake lies in a ravine-like hollow about 80 metres north of the Falkensteinkamm ridge, which carries the Austrian border, and some 500 metres south of the lower Weißensee. It is fed by groundwater, and its outflow, the Faulenbach, carries hydrogen sulphide that is noticeable by smell. Diving has been banned since 1983, with permits only for scientific purposes.

The water holds one of the world's largest concentrations of purple sulfur bacteria, forming a pink layer between 15 and 18 meters deep. These microscopic organisms develop under special conditions and make this place unusually interesting from a biological standpoint.