We already have a very secure underground shelter/survival area, here on the farm. On the 2-3 occasions we’ve had to go there, the tornadoes missed the compound by 1+ miles, so we were fortunate.
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.