The South Dakota shelters are old munitions bunkers, meant to contain an explosion from inside. Not, defend the bunker from an outside major explosion. About a pound of C4 plastic explosive would open the door. Main weakness is lots of small bunkers with no method of defense of the whole. The larger communal shelters have a major weakness -- the guards and techies. Both those see the wealthy with their pretty women and goodies. Why guard that, when you can have that. Billionaires better include people who can shoot, repair equipment and have medical degrees.