Concentric Spheres (Hollow Earth theory)

Concentric Spheres and Polar Voids is a hollow earth hypothesis, credited to John Cleves Symmes (as in Symmes's Theory of Concentric Spheres), who first proposed it in the early nineteenth century. It holds that the "earth is hollow and habitable within; containing a number of solid concentric spheres, one within the other, and that it is open at the poles twelve or sixteen degrees."