Along the same lines....
I attended the NATO school in Oberammergau many moons ago. Oberammergau is a picture-postcard German village nestled in the mountains. Just down the road from the school gates (a former German Army barracks) there is an overgrown road leading off into the trees towards the base of a mountain. Following the road takes you past a stereotypical dilapidated guard hut and barrier.
The road ends at the base of the mountain where the entrance to a 262 engine factory was located. The entrance has been destroyed to prevent "exploration" (apparently some kids got hurt in the past and there are parts that are flooded).
On the other side of this mountain however (on the road to Garmisch-Patinken), right next to the road and railway are two big metal doors. Apparently this was to allow the finished products to get loaded straight on to a train. Fascinating stuff and I believe that location was known as "Dora" and a documentary was done (scuba diving was required).
When I asked around I was told it was one of many "underground" manufacturing sites. It makes you wonder just how many are undiscovered so to speak.