Wikipedia
Seegrotte
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.