I worked for years in a small-town post office up north that had been built in the 1950's. One day, I accompanied the post master into the basement (which I did not know was there) to retrieve some archived files, and was stunned to see that it was a fall-out shelter! It was heavily fortified with steel doors and had those bright yellow nuclear-bomb warnings all over the inner doors. There were hundreds of barrels in the corners marked with "biscuits", "water", "masks" and such. The basement was huge, too; five hundred people could have easily fit with plenty of elbow room.

Another thing about that building: it had double walls. There was outer wall as seen from the street; then, there was a narrow hallway all around the perimeters of the building, between the inner walls and the outer walls. There were square peepholes set into the inner wall that would look out onto the post office work floor. Looking through those peep holes, one could see everything that was going on. When you went onto the work floor and tried to spot the peep holes, they were so close to the ceiling that they were nearly unidentifiable, unless you knew what you were looking for.