In the place called Hinterbrühl which is located some 17 km from Vienna, Austria there is an interesting tourist attraction, Hinterbrühl Seegrotte. Before 1912 it was used for extracting gray and red gypsum. Gypsum was used in agriculture as fertilizer. The mine was flooded in 1912 due to an accident and 20 million liters of water to flushed in creating the largest underground lace in Europe. After that the mine was used for public tours. During second world war German Nazi opened an underground factory producing "Heinkel HE 162 Salamander", the first jet fighter in the world. This plane was one of the secret weapons of the german Luftwaffe. There is a model of the plane as well as some parts of it.
It is quite cold inside. The temperature stays around 9°C throughout the year. After a short excursion through the galleries, there will be a bout tour on the lake. It is very nice and quiet there. the whole tour will take about an hour and costs 11 Euro.