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 litres of water.

It became a tourist attraction after 1930 and has been one ever since, with the exception of World War II.

[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".