On April 10, 1818, John Cleves Symmes, Jr. proclaimed his belief that the earth was hollow and habitable within. He did so by publishing a pamphlet and sending it to every college and scholar in America. Symmes wasn't hoaxing anyone. He was deadly serious about his theory. So why is he included in the Museum of Hoaxes? Because his ideas probed at the outer edges of his society's concept of reality, in the same way that hoaxes do.
This picture comes from an account of Symmes's theory prepared under the direction of his son and devoted follower, Americus Symmes. Note the shadows of successive spheres visible within the globe. This depiction of 'spheres within spheres' actually contradicts what Symmes was teaching during the final three years of his life (he died in 1829). At that point, he believed that only one hollow earth existed, not a number of hollow spheres.
If you're ever in Hamilton, Ohio check out the memorial that Americus Symmes erected there in the 1840s to honor the memory of his father.