Esoteric or quasi-scientific theory that the planet Earth is actually hollow, almost invariably including the idea that the inner surface is habitable.
There are two kinds of Hollow Earth theories: Concave Hollow Earth theories and Convex Hollow Earth theories. Convex Hollow Earth theories state that the surface of the planet we live is the external surface of an hollow planet. Concave Hollow Earth theories state that we and all the universe are inside a hollow planet.
In 1818, John Symmes suggested that the Earth was actually a hollow shell about 800 miles thick, with openings at both poles about 1400 miles across. With charming candor he proposed calling the "inner lands" Symzonia. Edgar Allan Poe used the idea in his 1838 The Narrative of Arthur Gordon Pym.
Jules Verne, who normally did not stray far from the bounds of scientific plausiblity in his works, used the idea of a hollow Earth in his 1864 novel, A Journey to the Center of the Earth, and Edgar Rice Burroughs, more concerned about entertainment than plausiblity, also wrote tales of adventure in the inner world of Pellucidar (including, at one point, a visit from his character Tarzan). Burrough'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.
Other writers have proposed that subterranean caverns or a hollow Earth are the abodes of "ascended masters" of esoteric wisdom. Antarctica, Tibet, Peru, and Mt. 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.