The Seegrotte Hinterbrühl is an underground cave system in an old gypsum mine just south of Vienna, Austria. The mine flooded with 20 million liters of water in 1912 and opened as a tourist attraction in 1930 when water levels were adjusted by pumps. It sees tourists to this day, though during WWII the caves were used as underground jet aircraft factories. In 2004, five tourists drowned when the tour boat overturned. I recommend it as an interesting side-trip if visiting the old Imperial capital.