Google house plans of the 1950s w/ bomb shelter. Both my grandpas built their houses in the early 50s, both have a bomb shelter built into the basement.
Only my maternal grandpas would be liveable for an extended period of time, with a water supply , shower , toliet, fireplace , wood storage, & food storage area included.

Both have reenforced concrete ceilings , walls , & triple angled hallways to get into them.I dont know what the entry doors are made of.

Both grandfathers houses are side by side & when i was young I always thought putting a tunnel underground to connect the two would be cool . They didnt think so.