Reminds of a a two hour show, either on the Smithsonian Channel or the History Channel, that covers some of the underground areas in Germany from the war, including factories built into mountains. A lot of it was sealed up by the allies after the war and then forgotten. They went into a few, and there were rocket motors, housing, etc. just left there.
Read Pynchon’s “Gravity’s Rainbow.” He really did his research!