Seegrotte

The Seegrotte, near Hinterbrühl, Austria, is a 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.
World War II history.
During World War II the Seegrotte was used for production of Heinkel He 162A jet fighters in Nazi Germany's "second Ruhr".