how badass to discover that buried in your backyard. hope he wore the proper gear, that's probably not fiberglass, i'd wager it was some sort of asbestos containing material
I am pretty sure that it WAS fiberglass. A lot of those shelters were pre formed fiberglass. You dug a big hole, put the form down, poured concrete and then you are done. There is a pic where he is removing the fiberglass and it shows that the base of the shelter is poured onto it.
The fact that it wasn't filled with water and perfectly dry after all that time AND in pretty good shape is a testament to the build quality. Very cool.