While Jules Verne may have been the most famous writer to expound on the concept that the Earth is hollow, Journey to the Center of the Earth was explicitly a work of fiction.
Early-1800s lecturer John Symmes Jr., however, wanted to let people know that Verne's visions were not as fantastic as they seemed. Ohio's Hollow Earth Monument honors the man's spurious science.
Symmes' Hollow Earth Theory posits exactly what you'd think: that the Earth is in fact hollow. According to Symmes, the empty center of the planet is accessible via shafts located at the north and south poles of the planet, as though Earth is some sort of celestial jewelry bead.
While the theory seems far-fetched by modern standards, Symmes was able to garner a strong amount of interest in the concept via his lecture tours, where he displayed his research into the magnetic fields that he claimed were proof of the holes at the poles. Symmes garnered so much interest that he actually got Congress to vote on funding that would allow him to mount expeditions to the polar regions in the 1820s, where he guaranteed they would find the entrances to the center of the planet. Unfortunately for him, the government did not share Symmes' sense of wonder and the grant was voted down.
After the rigors of the lecture circuit took their toll, Symmes retired to Hamilton, Ohio, where he would eventually died in 1829. One of Symmes' acolytes, Jeremiah Reynolds, continued the Hollow Earth cause for a time, even finding a ship to take him to Antarctica in search of one of the entrances to the inner Earth. Nothing was ever found.
Symmes is remembered by a monument in Hamilton, Ohio's Ludlow Park, which features an abstract hollow Earth, atop a stone pedestal and a plaque that explains his theory. Quackery or not, Symmes' sci-fi theories will not be forgotten anytime soon.
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