#23 It was not a fall out shelter... : )
A septic tank.
“...mucking out all the mud and spending time down there”
A previous owner repurposed the bunker to serve as a cistern. A French drain and my pond are plumbed to overflow into it.
Thus, it fills up with water over the course of a year (or in the case of a hurricane, over the course of a day).
I got 40 inches where I live!