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.[1] [2] The lake is 60 meters below ground, the water surface is 6200 m² and pumps are used to keep the water level down.[2]
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 162A jet fighters in Nazi Germany's "second Ruhr".[1] [3]
. Adolf Galland . 1968 . Ninth Printing - paperbound . The First and the Last: The Rise and Fall of the German Fighter Forces, 1938-1945 . New York . Ballantine Books . 237 . 1954.