According to Newton's Law of Gravity, the gravitational force is actually zero inside a spherical hollow shell of matter (absent other masses). Thus, even if the Earth were hollow, someone on the inside would not be pulled outwards or be able to stand on the inner surface, as is popularly supposed; rather, they would be weightless (with some slight residual gravity arising from the fact that the Earth is not perfectly spherical). (The centrifugal force from the Earth's rotation would pull a person outwards, but even at the equator this force is only 0.3% of ordinary Earth gravity.)

In ancient times the idea of subterranean realms seemed arguable, and intertwined with the concept of places such as Greek Hades, Jewish Sheol, and Christian Hell.

In 1818, John Cleves Symmes, Jr suggested that the Earth was actually a hollow shell about 800 miles thick, with openings at both poles about 1400 miles across. With charming naivete he proposed calling the "inner lands" Symzonia.

Other writers have proposed that subterranean caverns or a hollow Earth are the abodes of "ascended masters" of esoteric wisdom. Antarctica, the North Pole, Tibet, Peru, and Mount Shasta in California, USA, have all been suggested as the locations of entrances to these subterranean realms, with some advancing the theory that these places are the actual homeland of UFOs.

One such idea was promoted from 1945-49 as "the Shaver Mystery" in the pages of the science fiction pulp magazine "Amazing Stories." The magazine's editor Ray Palmer ran series of stories by Richard Sharpe Shaver, claiming they were factual, though presented as fiction. Shaver claimed the earth was honeycombed with caves that had been built by a superior pre-historic race, and that their degenerate descendants, known as Dero, live there still, using the fantastic machines abandoned by the ancient races to torment those of us living on the surface. One characteristic of this torment was "voices" that came from no explainable source. Thousands of readers wrote to affirm that they, too, had heard the fiendish voices from inside the earth.

Concave Hollow Earths

Instead of saying that we live on the surface of a hollow planet, sometimes called a "convex" hollow-Earth theory, a few others have opined that our universe is itself on the interior of a hollow world, calling this a "concave" hollow-Earth theory. Neither type of speculation is generally taken seriously by scientists.

One such proponent was Cyrus Teed in 1869, a self-described alchemist who founded a cult called the Koreshians based on the notion.

At least one proponent of a Concave Hollow Earth theory actually proposed new laws of physics to deal with the gravitation problem. Martin Gardner discusses the new model in one chapter of his book On the Wild Side. According to Gardner, this theory states that light rays travel in circular paths, and slow as they approach the center of the spherical star-filled cavern. No energy can reach the center of the cavern, which corresponds to no point a finite distance away from Earth in the widely accepted scientific cosmology. A drill, Gardner says, would lengthen as it traveled away from the cavern and eventually pass through the "point at infinity" corresponding to the center of the Earth in the widely accepted scientific cosmology. Supposedly no experiment can distinguish between the two cosmologies. Martin Gardner does not accept the Concave Hollow Earth Theory, and suggests that the man who proposed it may have had religious motives.

Hollow Earth in Fiction

Jules Verne, who normally did not stray far from the bounds of scientific plausibility in his works, used the idea of a hollow Earth in his 1864 novel, A Journey to the Center of the Earth.

Edgar Rice Burroughs, more concerned with entertainment than plausibility, also wrote tales of adventure in the inner world of Pellucidar (including, at one point, a visit from his character Tarzan). Burroughs's Pellucidar is notable for the fact that, although the inner surface of the Earth is of absolutely smaller area than the outer, those areas which are oceans on the outer surface are continents on the inner and vice-versa, so that Pellucidar actually has a greater land area than the "outer" continents combined. It is also inhabited by primitive humans and by an exciting mix of all those large and dangerous creatures which have unfortunately become extinct on the outer surface, and to which Burroughs did not hesitate to add such improvements as the Mahars, creatures vaguely resembling large intelligent pterodactyls with dangerous psychic powers. Pellucidar is lit by a central miniature sun which never sets, so that the human inhabitants have never developed the notion of time.

In the science-fiction novel Hard To Be God, written by the Russian authors Boris and Arkady Strugatsky, an Earthling space traveler lands on a planet where, due to an atmospheric peculiarity, the native population is convinced that it resides inside a concave hollow earth. As a result, they cannot accept his interplanetary origin.

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