Umberto Eco considers Outlandish theories: Kings of the (hollow) world. (Hat tip, Dave Lull.)
I remember seeing , when I was a kid, a Gene Autry serial that featured, mirabile dictu, an army of horsemen from a civilization that lived underground. Any codgers out there besides me who recall this?
Update: Thanks to Google I have found the name of that serial: The Phantom Empire. The Thunder Riders "disappear 25,000 ft. below the earth's surface to the 'Scientific City of Murania,' an underground empire lorded over by Queen Tika (Dorothy Christy), a blonde Amazonian who constantly compares her superior society with that of the pitiful world above. "
Hah! And you thought I was becoming delusional! There's more here and here. Wacky I'm sure it is, but it says something that it has stayed in my memory all these many decades. I must have been a more impressionable child than I thought.