From the article "Underground" in GURPS All-Star Jam 2004

Military officer John Symmes hypothesized a hollow earth, accessible by a large open area near the Poles. Symmes rejected newtonian gravity He other wrote or wwas satirized by the novel Symzonia, where the hollow earth is inhabited by a utopian society, run by a benevolent aristocracy, which has long since banished all of their misfits to a distant colony, and possibly the outside Earth. The Symzonians have advanced airships, boats with jet propulsion, flame-proof cloth woven from spiderwebs, and a mysterious weapon of mass destruction.

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