Seegrotte Hinterbrühl

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 litres 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 water level down. Visitors can tour the old mine and take a boat ride across the underground lake.

Mine data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).