Failed Hollow Earth Utopia
Cyrus Teed was a nut who believed that we lived on the inside of the Earth.
He was evidently a persuasive nut, for by the turn of the 20th century he had convinced several hundred people to move with him to an empty wilderness on the Gulf Coast of Florida, to build what he confidently predicted would become a city of ten million followers. Teed called his cult "The Koreshan Unity" (he renamed himself Koresh) and while the group had some noble beliefs, they also had a lot of crazy ones, like the inside-out Earth thing. He thought that centrifugal force held us on the ground -- gravity was a bunch of hooey -- and the sun was actually powered by batteries. When Teed died in 1908, his disciples kept him in a bathtub for several days, expecting that he would rise from the dead. He didn't, and the cult began its slow decline.
The last four survivors deeded the property to Florida in 1961. Its few remaining buildings are maintained as a state park, and its highlight is a "Tootsie Pop" model of the inside-out world as conceived by Teed.
Ten million people probably do live in this region now, although theirs is a utopia of strip malls, golf courses, and million-dollar homes.