I've always been fascinated by subtereanean housing!
In the early 60s there was a thing known as the Cuban Missle Crisis. One of the primary targets was an Air Base ~25 miles away. We were taught in grammar school to put our heads under our desks in case of an atomic attack. Lots of base employees had bomb shelters built in their back yards. Most were very discreet, with hidden entrances disguised as BBQs, doghouses, etc.
The crisis blew over, and many of them were filled in, but there are a few left. Not too long ago, my brother was called to sell a property and landed a contract with a family that just loved the house. I was called to open the garage that had not been opened in a LONG time. The new owner mentioned that the yard had the ugliest brick BBQ he's ever seen, and how could it work without a chimney? Upon closer inspection, I found a heavy steel door on the brick structure. Thinking it to be a "smother" door for an ill-designed BBQ, I pried it open.
Lo and behold, there was a set of rungs leading ~15 feet under the yard, leading into a corridor to another steel door, behind which was a 12 foot square concrete room with a 7 foot ceiling, shelves, cabinets, empty 55 gal. drums, a stationary excersise cycle, set up to operate either a 12 volt generator or an air pump, a hand-operated air pump, and 12V lighting. This enclosure had not been entered in over 25 years, and there was NOT ONE bug or cobweb in the whole place; dry as a bone.
Upon further inspection, we found the 2" iron pipe going up to a hidden air filter at ground level, and a blocked off set of stairs leading INTO the house. To say the new owner was blown away would be a major understatement!