The destination is an underground lake dug out of a small mountain to mine gypsum. A very special destination. A 400 metre walk with guide horizontally into the centre of the hill passes some representations of rest areas for the miners and accommodations for the blinded horses that spent all of their 20 years' working life underground, in the dark and toiling hard. Must have neede the gypsum real bad!
The result of all that mining produced a huge underground area with lakes at three levels. In the middle of these we took a motorboat ride with our guide. Quite a surreal experience!
This underground space contains a theatre where the Bienna Boys' Choir performs regularly, and where an opera is currently running.
In WW2 the Nasty Nazis used slave labour supplied from the small concentration camp above to build an extraordinarily unsuccessful jet fighter with its engine mounted on top. Seems that everything about it worked well apart from landing the bit. Most of the frame was made of wood(!)