There are many caves and caverns throughout the world.
Would not a shelter built in a cave be far better than one above ground ?
The US Government and agencies has a plethora of underground bases and facilities.
Ther are not enough blast free or fallout free zones in the United States for many to survive above ground, except in special shelters.
In special shelters water and food runs out, the occupants go insane and kill each other, plus there is no TV. :*>
In your own private underground cave shelter YOU control , not some armed guard or government agent.
If you are deep enough in the ground the fallout that settles on the surface cannot harm you.
You must filter the air to prevent fallout from entering your shelter area.
If the fallout particles are caught a safe distance away in air vents then the effects are kept away.
In areas of known caves and caverns, even if there is heavy fallout, your best survival chance may be deep underground. Some will go there .
SOME WILL SURVIVE.