@djjd62,
I've read the history from what I could gather on it as well, the folks who were pushing it back in the day weren't at all disreputable.
As with most things, I believe it may be a combination of fiction tales, but those tales are roughly based on some kind of truth or experience we can't explain or even see.
For example: there have been hidden shafts discovered in south america leading into unknown caverns and others, apparently inhabited at one time by the inca and other civilizations.