23-35. While an underground shelter covered by 1 meter (3 feet) or more of earth provides the best protection against fallout radiation, the following unoccupied structures (in order listed) offer the next best protection:
Caves and tunnels covered by more than 1 meter (3 feet) of earth.
Storm or storage cellars.
Basements or cellars of abandoned buildings.
Abandoned buildings made of stone or mud.