Cyrus Teed was a bit of a kook. Born in 1839, he became an eclectic physician and established a laboratory in which he carried out alchemical and electrical experiments. After one particularly notable experiment in 1869 (of which there are varying accounts: one says he turned lead into gold; another claims he was shocked to within an inch of his life), Teed saw a vision of a beautiful woman who revealed the secrets of the universe to him. After this experience, he changed his first name to "Koresh" and established a rather odd cult (ahem, I'm sorry, utopian community) based on the principles of Koreshanity.
The most salient principle of Koreshanity was a belief in a variant of the Hollow Earth theory, which can basically be summed up by saying that the world in which we live, and indeed the entire universe, are all contained within a sphere -- and we live on the INSIDE of the sphere, with centrifugal force holding us down, rather than gravity. Oh, and the sun is actually a giant battery in the center of this giant sphere. And alchemy works; Teed claimed to have discovered the Philosopher's Stone. But anyway, here's a snippet from Teed's Cellular Cosmogony:
"The sun is an invisible electromagnetic battery revolving in the universe's center on a 24-year cycle. Our visible sun is only a reflection, as is the moon, with the stars reflecting off seven mercurial discs that float in the sphere's center. Inside the earth there are three separate atmospheres: the first composed of oxygen and nitrogen and closest to the earth; the second, a hydrogen atmosphere above it; the third, an aboron (sic) atmosphere at the center. The earth's shell is one hundred miles thick and has seventeen layers. The outer seven are metallic with a gold rind on the outermost layer, the middle five are mineral and the five inward are geologic strata. Inside the shell there is life, outside a void."
Teed established various settlements but his crowning achievement was his "New Jerusalem," also known as the Koreshan Unity Settlement, in Estero, Florida, near the city of Naples. It was founded in 1894. About 250 residents followed him to the settlement and constructed various buildings, including a print shop (where they published a newsletter called Flaming Sword), a post office, an early power plant, and the World College of Life. Residents used a sort of tagline, "We live inside," to denote their belief in the hollow-earth principles of Koreshanity.
On December 22, 1908, Teed died. His followers, who believed in reincarnation, left him in a bathtub awaiting his second coming -- which logically would happen on Christmas day. When no reincarnation occurred, local authorities forced the corpse to be buried (on December 27), and that marked the beginning of the end of the Koreshan Unity Settlement -- although it would survive more than 50 years past Teed's death.
In 1961, there were just four colony members left. Their leader, Hedwig Michel, deeded the 300 acre utopia to the State of Florida. It is now the Koreshan State Historic Site, an area of just 135 acres in which, as the Florida State Parks Information Center puts it, "visitors can fish, picnic, boat, and hike where Teed's visionaries once carried out survey experiments to prove the horizon on the beaches of Collier County curves upward."
Further reading: RoadSide America's write-up, official Florida State Parks site, and an unofficial site which includes tons of information (check out the art collection, bio of Cyrus Teed, and an essay on Teed and the Koreshan Unity Movement).