Subbterranea Britannica 
documenta underground structures - from WW II and the Cold War. Here are images from various shelters in London:
Nick Catford wrote, in August of 2004,
After nearly eight years and thousands of miles the survey of 1563 ROC underground monitoring posts finally came to an end on Monday 16th August when the last post site was visited at Port Ellen on the Island of Islay off the Scottish west coast.
The folowing set are of disused Royal Observer Corps posts - they’re all artifacts of the Cold War, acording to Subterranea Britannica - none has a construction date early than 1957; they were all closed in September of 1991:
Note that none appears hidden - at least not based on these relatively recent images. Nor does any have a gun port - or multiple gun ports, which could create a field of fire.
Subterranea Britannica: the study and investigation of all man-made and man used underground places.
I’d like to see the tunnel that Sherlock Holmes discovered in the “The Red-Headed League.”
