Seegrotten maanalainen lentokonetehdas Hinterbruhl

The Seegrotte, near Hinterbrühl, Austria, is an underground cave system with a large grotto located under a former gypsum mine.

It was closed in 1912 after the mine flooded with 20 million liters of water. It became a tourist attraction after 1930 and has been one ever since, with the exception of World War II. The lake is 60 meters below ground, the water surface is 6200 m² and pumps are used to keep the waterlevel down. 

Visitors can tour the old mine and take a boat ride across the underground lake.

During World War II the Seegrotte was used for production of Heinkel He 162 jet fighters in Nazi Germany's "second Ruhr"

 

In 1944 Ernst Heinkel went on a search for a bombproof production site for the  He162 "Salamander", the choice fell on the lake grotto. The underground lake was drained, the floor filled with concrete and the necessary equipment installed (lighting, heating.

Despite the terrible situation as a whole in 1944/45 could be made in the tunnels 198 fuselages of the "People's hunter". Employees were over 2000 people, including 1700 KZ prisoners from KZ Hinterbrühl. The final assembly was carried out in Schwechat - Heidfeld in Heinkel factory at today's airport