Weingut 1 (Vineyard 1) was the code name of a subterranian arms industry installation of the Nazis in Bavaria. It has never been finished and after the war partially destroyed.
https://de.wikipedia.org/wiki/Weingut_I
Today it is a labyrinthic mass of large concrete structures with a lot of rusty iron rods overgrown by thick moss and forest. And you can find subterranian chambers with stalactites on their ceiling.

If you climb down into a very narrow gap in the concrete, you can enter this underground chamber.