Oh, the bottom half of the shelter is definitely below the water table here in Houston.
The whole room is intact and maintains its integrity, and thus floats a little. There is a rectangular outline in my lawn indicating that the entire structure has raised a few inches over time.
I was advised to keep it full of water to prevent that.
It was wired for electricity, and one of the ways I dated the structure was by dating the incandescent light bulb that was still in its fixture. In the old days, round incandescent bulbs had a small bump at the very top. It was not completely spherical.
Thus, my guess is that it was installed during or shortly after the Cuban Missile Crisis.
Sweet.
I want one that doubles as a rehearsal studio, I knew of one of those in San Francisco.