This very complex subterranean object in the Eifel had its origins in natural caves and a mine and was used in the 20th century for different purposes.
There is an area where the (fallen down) stones are marked with blacklight active paint - which inspires effectful blacklight photos.
Link
KrissieFox
So this is some sort of old mine? I saw eifel in the title of this photo set and first thought there was some sort of cave system under the Eifel tower...