The Hollow Earth theory is the idea that our planet is not solid, but instead contains vast interior spaces—or even a complete inner world. Versions of the theory have existed for centuries: some early scientific models in the 17th and 18th centuries speculated that Earth might be composed of concentric hollow spheres, while later occult and pseudoscientific writings imagined entire civilizations living inside, accessible through openings at the poles. Though popular in fringe literature and adventure fiction, the theory has no scientific basis—modern geology and seismology confirm that Earth has a solid mantle and core rather than a hollow interior. Still, the Hollow Earth concept remains a lasting curiosity in myth, conspiracy lore, and science fiction storytelling.