A belief unique to the religion of Koreshanity. The theory of the Cellular Cosmogony states that the earth is a hollow, concave sphere, and that mankind is inside it. Thus, the other side of the planet is above you, and the sun, moon, and stars (actually very small, though bright) float in the center of the sphere. According to the theory, gravity does not exit -- we are held in place by centrifugal force.
A team of Koreshan scientists once "proved" this theory by running carefully-tuned measuring instruments across a mile-long stretch of Naples Beach in Florida. They reported a subtle incline, satisfactory verifying for them that the earth curved upward.
Reports of WWII-era Nazis experimenting with similar ideas (independent from the Koreshans) tell of spies directing high-powered telescopes upward in hopes of seeing the Allied forces. It's unknown whether these stories are true or apocryphal.