Of course the site is conveniently located on the China Lake naval gunnery range, or some such installation in Death Valley. (I know this because this was in a book called Mysterious California).
You'd have to be nuts to accept this tale at face value, since the enterance has mysteriously disappeared. Speaking of nuts, it is alleged that
Charles Manson intended to lead his followers into such an underground world after starting a cataclysmic race war (can't back that up, neither could
the book where i read it).
The most likely explanation is that these are the burrows of a race of highly evolved nuclear red ants which I flushed down the toilet 10 years ago
after finishing my 6th grade science project.