The Hollow Earth theory proposes that Earth is entirely or partly hollow inside, sometimes featuring giant polar openings or hidden inner worlds.
Edmond Halley (1692): The famous English astronomer proposed that Earth had a shell about 500 miles thick with inner concentric spheres to explain shifting compass readings. He guessed these spaces might hold glowing atmospheres or life.
John Cleves Symmes Jr. (1818): An American army captain who claimed the planet consisted of multiple hollow concentric shells with massive openings at the North and South poles.
The concept inspired classic adventure novels like Jules Verne's A Journey to the Center of the Earth and Edgar Rice Burroughs's Pellucidar series. Later believers claimed that UFOs, secret Nazi bases, or advanced lost civilizations reside inside the Earth.
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