I’m trying to build one myself right now. 10x12, 3 ft. of dirt on top, 8 inches of concrete and rebar. Kind of a dual purpose. Two big bulls eyes in this area, and super cells overhead for the last two years.
This is what Dad & I built, but 2x as large, several years ago (scroll down), and nicely furnished/well-stocked:
http://www.atlassurvivalshelters.com/aboutus/nbc/backyard/
“Two big bulls eyes in this area, and super cells overhead for the last two years.”
A couple of big ones just missed our 200ac farm by 1+ miles, so I know the damage it can wreak, as evidenced by the rescue/recovery ops we did.
Take care and be safe, c...