Schaubergwerk Seegrotte Hinterbrühl Hinterbrühl · Niederösterreich · Austria

Gypsum deposits in the area of Hinterbrühl in Lower Austria, which also occur at Preinsfeld near Heiligenkreuz, were known from the eighteenth century. In 1848 a miller in Hinterbrühl, sinking a well, struck gypsum at a depth of five metres and began extracting fertiliser gypsum. Within three years — by 1851 — the workings had developed into a mine of some complexity, with a main adit, subsidiary tunnels, and shafts, and there were plans to install an underground railway for materials transport. During the active mining period, horses hauled loaded wagons from the workings to the surface via the main tunnel; an underground stable and a chapel dedicated to St Barbara, patron saint of miners, were established within the mine.

On 26 June 1912, blasting operations caused a catastrophic inrush of water into the mine. More than 20 million litres of water — over 20,000 cubic metres — flooded the adits and tunnels, making further mining entirely impossible. The mine was abandoned. Over the following years only safety works were undertaken in the flooded tunnels. The inrush created a permanent underground lake at a depth of 60 metres below the surface, fed by seven underground springs and having no natural outflow; to maintain the water level, between 50,000 and 60,000 litres are pumped out nightly into the Mödlingbach. The lake surface extends across 6,200 square metres and averages 1.2 metres in depth. This body of water is designated Europe's largest underground lake.

In 1918 the flooded mine was purchased by Friedrich Fischer, a Viennese liqueur manufacturer. Fischer's interest in the underground spaces extended beyond mining; attempts were made to use the tunnels for leisure purposes and for cultivating mushrooms, but these proved unsuccessful. In 1932 the installation was opened as a show mine — the "Seegrotte" — equipped with a former military pontoon and an electric boat for the underground lake. The attraction drew substantial visitor numbers: in 1937 and 1938, approximately 50,000 guests visited the tunnels and the underground lake. Entry to the mine was via a 400-metre main adit leading to the underground chambers.

During the Second World War the strategic value of the bombproof underground spaces was recognised by the German armaments industry. Following allied air attacks on the Heinkel aircraft plant at Wien-Schwechat, the Ernst Heinkel AG searched for a bombproof production site for the Heinkel He 162 "Salamander" jet fighter — a project previously considered for Retz. The Seegrotte was selected. The underground lake was drained and its floor concreted; lighting, heating, and the necessary industrial infrastructure were installed. The project was given the codename "Languste". Under the supervision of Heinkel workers from Germany, concentration camp prisoners — part of the KZ-Außenlager Hinterbrühl, a satellite camp of the KZ Mauthausen, given the SS codename "Lisa" — were forced to assemble the fuselages of the He 162 in the underground tunnels. The prisoners were housed in barracks at a camp on the Johannesgasse in Hinterbrühl. The core prisoner population at the camp numbered approximately 800, rising to around 1,800 at peak, with sources indicating the total number passing through may have reached 3,500 over the period 1944 to 1945. The prisoners — predominantly Polish, Soviet, and Italian nationals — initially worked in two twelve-hour shifts, later in three eight-hour shifts. 198 He 162 fuselages were completed in the tunnels; the completed structures were transported out via rails, conveyed to Wien-Schwechat, and there assembled for flight. Prisoners were also deployed in the manufacture of V2 rocket nose elements and components for the He 216 night fighter.

The KZ-Außenlager Hinterbrühl was dissolved on 1 April 1945 as Soviet forces advanced. Approximately 52 prisoners unable to march were murdered with benzene injections before departure; 1,884 prisoners were then marched on foot towards KZ Mauthausen, of whom only 1,624 reached the camp. 36 civilians from Hinterbrühl died in an allied bombing attack on the Heinkel production facility during its period of operation. In 1945, as German forces withdrew, the underground production installations were blown up, causing the tunnels to flood again. In 1946 the bodies of murdered prisoners found in a mass grave on the former camp site were exhumed and reburied at the Vienna central cemetery. A memorial site on the former camp grounds was established in 1989, when the "Verein zur Errichtung einer KZ-Gedenkstätte in Hinterbrühl" purchased the land; commemorative stones and information panels were installed, and annual memorial events have been held since. A memorial tablet in the show mine, erected in 1988, commemorates the concentration camp prisoners and prisoners of war who carried out forced labour at the Seegrotte in 1944–45. Parts of the He 162 and the former production installations remain on display within the show mine.

Visitor tours of the Seegrotte resumed in 1948. In 1984 the leaseholder retired and the descendants of Friedrich Fischer took over direct operation of the show mine. In 1993 parts of the mine served as a filming location for the Hollywood production "Die drei Musketiere" (The Three Musketeers). In 2004 a tragic boating accident in the underground lake killed five people; boat tours resumed after a short interruption. In 2019 the Seegrotte was temporarily closed due to structural deficiencies. Following comprehensive renovation and enlargement works under new ownership, the show mine reopened in 2022. The site had received over 20 million visitors in total by the time of its most recent reopening. The Seegrotte is open year-round, daily from April to October and on reduced days from November to March.