Maintain your bunker folks!
Over at Retronaught, found this photo essay on an old personal Cuban Missile Crisis (1961) era backyard bunker. It appears to be directly underground, about 600-800 sq feet for a small family. Judging that it is more than 50 years old, was not sealed, etc it isnt actually in that bad of condition. However if it had been sealed properly, and the supplies rotated every few years, it could be Bug-In ready today. As it stands now, about the best thing to do is fill it in with earth and close it off.
More pictures here