Amazing photos. This guy runs a blog describing his search for a buried tunnel he suspected beneath his property ... incredibly, he turned out to be right.
Looks like it was a well constructed Luftwaffe bunker.
See how long those corrugated steel barrel vaults last? 70+ years old and going strong! If he gets a coat of paint in there and cleans the place up a little it should be good for another 300 years!
I love the barrel vault design, it's nigh indestructible. I am actually planning on a similar length and diameter for the "Grand Redoubt" when I get around to building it. I think the "Hoag Shelter" design can be an excellent compromise to get more usable floor space.