wait until you see the Huge metal staples that once supported the tunnel roof inside.
There are loads of them.
The concrete lintel has a solid metal door beneath it complete with Superman sign on the front.

I do hope all this crap goes and isn't left there for posterity.
I won't be allowed into the center of the hill (not that I would like to go in anyway) as I don't have a Confind Spaces certificate but my camera is going in with someone who does ;)
PeteG