... dirt; lots and lots of dirt! That's the best way to ride out a nuclear blast. This is probably aftermath of one of the fifteen 'shots' during "Operation Teapot" during 1955 at the Nevada proving grounds. All sorts of 'blast-effects' were documented during this series of atomic bomb detonations; including what sorts of shelters worked best. We are looking at a reinforced concrete structure, mostly below grade, with a big mound of dirt piled on top.
note: don't be directly behind that door when the bomb goes off - it looks pretty thin and won't stop much of the 'hard radiation' x-rays and gamma rays coming from the instant of detonation.