Langdon Hole Underground Communications Facility. Sometimes called DUMPY/B at it acted as a backup for DUMPY/A, under Dover castle. It was commissioned and dug at the same time as DUMPY/A.
Planned in October 1942. and completed in two months by the 172 Tunneling Company, Royal Engineers. It also served as a hub for recieving cross-channel communications after D-Day.
The block-houses at the main entrances were destroyed by the council in the early 1980s and all traces of the tunnel were removed... Or so they thought.
There is an identical facility two miles to the east of this at Long Hill; also built at the same time, but the new land owners have collapsed the entrance.

Here's some pics from my visit last year. Starting with a plan found on the net.



Looking up to the main entrance (note the rail tracks):


At the bottom:


Looking along section A :


Ventilation passage and plant room:


Looking from A along section C towards D:


Looking down section D towards secondary entrance.


From D down section B towards A:


Looking down A from rooms, towards main entrance.


Near to the bottom of secondary entrance. (blocked)


Too much time on my hands.