The sea grotto is a former gypsum mine in the Lower Austrian market town of Hinterbrühl, about 15 km south of Vienna, which served as a show mine from 1932, but was last closed in 2019. The new owners (2020) announced a complete, extensive renovation – with the aim of reopening.
Seven springs feed the largest underground lake in Europe with an area of 6200 m². It does not have a natural drain. The height of the water level is maintained by pumping around 50–60 cubic meters of water into the Mödlingbach every night. On average, the lake is 1.2 m deep.
Second World War
In 1943 the German Reich began to move important warehouses and factories underground. Originally they wanted to store edible fats in the tunnels, but this failed because of the high humidity. Large quantities of technical fats were stored in barrels. Cultural goods were also stored to protect against war damage. During this time the operation continued as a show mine.
It was not until May 1, 1944, that the tunnels were confiscated and a Heinkelwerke aircraft factory was set up there under the code name Languste, because the site was relatively safe from bombing raids. This was proven by the first bomb hit on May 24, 1944, which caused no damage whatsoever inside. For the production of aircraft, the lake was pumped out and a flat floor surface was concreted. Tunnels from the second and third level, in the direction of Neugasse, which should be passable with cars, were never completed and filled again after the war.
The hulls of the Heinkel He 162 (also Salamander) were built in it as prototypes, transported out in parts and assembled on the grounds of today’s Vienna Airport in Schwechat. In December 1944, around 600 concentration camp prisoners were employed there under the supervision of around 170 skilled workers. The concentration camp prisoners were housed in the Mauthausen concentration camp branch and had to work in the tunnels under inhumane conditions.
One explores the tunnel on foot, and one learns something worth knowing over and over again. The tunnel has also been used as a film set. At the end you come to a beautiful lake, on which you can take a boat trip that is well worth seeing.