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The hollow Earth theory puts for the idea that the Earth is completely hollow or may have a hollow region inside. This idea was common in some ancient cultures and was evidenced in ideas like the hell in Christian literature or Sheol in Kabalistic works. This idea has also been explored numerous times in fiction including Jules Verne’s Journey to the Center of the Earth and in a series of novels by Edgar Rice Burroughs as well as in more modern literature.
The famous astronomer Edmond Halley – the same person who computed the orbit of Halley’s Comet – suggested a version of the hollow earth. He said that there actually three shells inside Earth approximately the diameter of Venus, Mars, and Mercury. The shells rotated independent of each other, had their own magnetic poles, had atmosphere, light, and possibly even life. His theory was an attempt to explain why compass readings were anomalous. According to him, the surface of the planet is a separate shell about 800 kilometers thick. Additionally, Halley thought that gas escaped from the inner worlds, and that this gas was the cause of the aurora borealis.
Others also believed in the hollow Earth theory including the American John Symmes. Symmes thought that the Earth was hollow, but that it could be reached through entrances at both the north and south poles. He believed that these holes were thousands of miles wide and wanted to make an expedition to the North Pole in order to prove his theory.
There are a number of criticisms of the hollow Earth theory. One of the major criticisms is the presence of gravity. The theory of gravity shows that objects clump together into roughly spherical objects. Also, the mass of the Earth is too large for the planet to be hollow. If the Earth was hollow, the gravity on Earth would be much less than it is. Also, the inner Earth would not have any gravity, or would have very little gravity.
Although the theory was discredited and almost completely abandoned for at least a couple hundred years, some still believe that our world is hollow. Marshall Gardner published a book in 1913 claiming that the inside of the Earth was hollow and housed a Sun; he also believed that the inside of the Earth could be reached through the north and south poles. More recently, the hollow Earth theory has been linked to Hitler and Nazi Germany.
Astronomy Cast has an episode on Earth you will enjoy.