Seegrotte Hinterbrühl Schaubergwerke GmbH
Hinterbrühl is located a few miles from Laab im Walde. It is noted for the Seegrotte/the Lake Grotto. This is a complex of caves that originally were gypsum mines. Blasting in 1912 released water from an underground stream, flooding the lower levels of the mines and creating the largest underground lake in Europe. During the final days of World War II, concentration camp inmates from Mauthausen were used as slave labor in the upper levels of the mines for the manufacture of the German jet fighter, the Heinkel HE 162. Of the 362 fighter jets built, 120 of them saw action but were too late to change the course of the war. Boats now carry tourists through the caverns of the Seegrotte.