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 as early as 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 6200 m² area largest underground lake in Europe. It has no natural outflow. The water level is maintained by pumping around 50-60 cubic metres of water into the Mödlingbach every night. The lake is on average 1.2 m deep.
Second World War
In 1943, the German Reich began to move important warehouses and operations underground. The original plan was to store cooking fats in the tunnels, but this was not possible due to the high humidity. Large quantities of technical fats were stored in barrels. Cultural assets were also stored to protect them from war damage. During this time, operations continued as a show mine.
It was not until 1 May 1944 that the tunnels were confiscated and used to house an aircraft factory belonging to the Heinkelwerke under the code name Lobster because the place was relatively safe from bombing. This was proven by the first bomb hit on May 24, 1944, which did not cause any damage to the interior. The lake was pumped out for aircraft production and a flat surface was concreted. Tunnels from the second and third levels, in the direction of Neugasse, which were intended to be accessible by motor vehicles, were never completed and were filled in again after the war.
The fuselages of the Heinkel He 162 (also Salamander) were built as prototypes, transported out in parts and assembled on the site 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 branch of the Mauthausen concentration camp and had to work in the tunnels under inhumane conditions.
You explore the tunnel on foot, and you learn interesting facts along the way. The tunnel has also been used as a film set. At the end you come to a beautiful lake, where you can take a boat trip that is well worth seeing.
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