Hollow Earth is a theory proposing that the planet Earth is entirely hollow or contains a substantial interior space. 
The crux of the hollow earth theory is that the earth is a shell with walls about 800 miles thick. In the polar regions there are holes 1400 miles across, with edges that curve smoothly from the outside of the shell around to the inside. A sea or surface traveler could proceed over an edge of the hole, like an ant crawling over the lip of a coffee mug from the outside to the inside, and not be aware that he was actually entering the interior of the earth. Dr. Raymond Bernard or Walter Siegmeister (we’ll talk about him later) explains that the holes have never been seen from the air because pilots are fooled by their compasses into believing that they are crossing the pole, when they are actually following the hole's "magnetic rim". Thus aircraft never really fly over the geographic poles, which naturally mark the centers of the holes themselves. As irrefutable proof of his claim, he cites Admiral Byrd's statement, "I'd like to see that land beyond the Pole. That area beyond the Pole is the Great Unknown."
Some conspiracy theorists believe the Earth is hollow—and that superior 'alien' humans, Vikings and Nazis live inside.

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