The Seegrotte cave system in the district of Mödling in Lower Austria is the result of mining for gypsum. An explosion during blasting in the mine in 1912 led to its closure, and also the creation of what is believed to be the largest subterranean lake in Europe. The cave system was ‘re-discovered’ by speliogists in 1930 and subsequently developed for tourism. Visitors go 60 m below ground level into an area of 6200 square metres where they can see the pool, which is fed by seven springs. They can explore the old gypsum workings, and cross the lake in boats. They can see the miners’ chapel dedicated to St Barbara, which was completed in 1864, and commemorates past generations of miners. Its altar is adorned with initials GA (Glück Auf: good luck) the traditional miners’ greeting.
Between 1883 and 1932 the caves were linked to the village of Hinterbrühl by one of Europe’s first electric tramways.
In 1943, during the Second World War a forced labour factory, a satellite of the Mauthausen concentration camp, was built in the upper levels of the cave system. Engines and sub-assemblies were built in the underground factory for the pioneering Heinkel He 162 Salamander jet fighter, but the aircraft were assembled at the airfield at Vienna Schwechat. A model of a Salamander aircraft is displayed in the workings, and there is a monument above the lake to 51 prisoners who died during a forced march of 200 km from other camps to Mauthausen Hinterbrühl in April-May 1945.