The Seegrotte in Hinterbrühl holds 20 million litres of water inside a former gypsum mine, making it the largest accessible subterranean lake in Europe. The cavern flooded in 1912 when a single dynamite blast ruptured an aquifer, drowning the mine in minutes and ending a century of plaster production that had supplied Vienna's Baroque palaces. The workers escaped, but the mine never reopened. Instead, the drowned shafts became a clandestine aircraft factory during the Second World War, where forced labourers assembled Heinkel He 162 jet fighters under a thirty-metre rock ceiling, hidden from Allied reconnaissance. Today the lake sits silent and still, its surface reflecting limestone ribs and distant tunnel mouths. The water temperature holds steady at nine degrees Celsius year-round, insulated by the surrounding massif. No fish live here; the aquifer is oligotrophic, carrying almost no nutrients. The only movement comes from the electric boats that ferry visitors across the main chamber, their hulls cutting thin wakes through water so clear it appears black in the low light. The cavern's acoustics are peculiar — voices carry across the lake but dissipate quickly, absorbed by the stone. Vienna underground lake tickets grant access to a fifty-minute guided circuit that begins above ground, descends through hand-cut corridors, and culminates in the boat crossing. The tour route follows the original miners' paths, widened and lit but otherwise unchanged since the nineteenth century. Wooden props still brace the tunnel ceilings in places, and you can see the pick marks where crews worked seams of selenite, a crystalline form of gypsum prized for its translucence. The mine's wartime history is documented in a small exhibition near the entrance, with photographs of the assembly lines and testimonies from survivors of the labour programme. The lake itself spans 6,200 square metres, its depth varying from two to twelve metres depending on seasonal aquifer recharge. The main chamber reaches sixty metres in length, and the ceiling vaults rise to fifteen metres at the apex. The stone is Wetterstein limestone, deposited during the Triassic period when this region lay beneath a shallow tropical sea. Fossils of ammonites and crinoids are visible in the tunnel walls, their forms frozen in calcite. The Seegrotte remains one of the few underground lakes in Central Europe where visitors can travel by boat rather than merely observe from a platform, a consequence of the mine's unusual geometry and the stability of the surrounding rock.