Speaking of the Wild West and cavern systems, Mary Martin's paperback The Best of the Hollow Earth Hassle contains some interesting material. Various writers contribute. It seems that they were the vanguard of the hollow earth, underground worlds movement in the seventies, and this was just a bit after Raymond Bernard (Zigmeister?), who wrote the book The Hollow Earth, disappeared in Brazil.
For example, Mary Martin wrote Chapter Eleven herself and it was/is about cave systems in Wyoming. There is a nice picture of a Sphinx in Wyoming in that chapter.
This just goes t show that you don't have to go the the Andes to find underground tunnel systems and such.
By the way, Wyoming - a state of the union - was named after a Chinese emperor of the Ming dynasty, Yao Ming. It wasn't intentional, I'm sure, but I am not kidding.
Her book is available and it is not expensive.
I think she is on the list, even.
And ..., in the same region of the country:
Ancient Civilization beneath Death Valley:
Author unknown Bourke Lee, in his book 'DEATH VALLEY MEN' (MacMillan Co., N.Y. 1932), chapter:
"Old Gold", describes a conversation which he had ...
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And the Apache used underground tunnel systems near Area 51, and defended the place where an opening was:
Apache Underground! Below, the reader will find two short excerpts from the book Lost Cities of North America, by David Hatcher Childress.
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David Hatcher Childress has written quite a bit about the connection between native American indians of the Southwest and cavern worlds below in his books; specifically, Lost Cities and Ancient Mysteries of the Southwest.
I don't know why Childress isn't more popular and celebrated on the internet, he has brought so much about lost and unknown civilizations to our attention.
The link from the first page of this thread is still good: An Underground City of Giants Discovered in the Grand Canyon | Archaeological Hub
And I have just shamelessly lifted this link from Gary Fleck's Facebook hollow earth page: https://treasuresinamerica.com/adventures-treasures/underground-egyptian-city-in-the-grand-canyon/?fbclid=IwAR0eWyWPcAyPAvKzRkSMDMFTDtaMewqnDF4nvEIBWAt1j92d0V7nNHruvo8
Is the picture at the top from Arizona or Egypt? Because, if it's from Arizona, I'm going to fall out of my chair!