The Flat Earth Theory is idiotic, isn't it? If you're offended by that word, don't be. People who believe it are nincompoops, cretins, nitwits, ignoramuses! Real idiots...
Well, for all the nonsense of the Flat Earth Theory, take a gander at the Hollow Earth Theory. It claims that the Earth is a hollow sphere, with humans living on the inner shell. (Not a typo: the INNER shell!) Inside the sphere... who knows? Hell, perhaps? Or the rest of the universe? It depends who you ask.
One group you could no longer ask is the Koreshean Unity (nothing to do with David Koresh), an utopian community started in the 1870s. They eventually made their way to Florida (it's always Florida) with the goal of building "New Jerusalem" (it's always Jerusalem).
The KU was started by a self-proclaimed messiah (is there any other kind?) named Cyrus Teed. It was eventually led by a Hedwig Michel, a German-born Jew who got out of Nazi Germany only to embrace the Koresheans. When Michel died in 1982, she was the last member of the community.
You see, the KU also believed in celibacy. Not exactly a good idea if you want your cult to be long-lasting...
Talk about idiotic!