|Some dare call it|
The Hollow Earth theory is a pseudoscientific belief and conspiracy theory stating that the Earth is hollow. Theories include that the inhabitants of the "Inner Earth" walk around the inside feet-up, or that a smaller sphere is inside the larger (sometimes it is stated this object works like a sun for the inner earth denizens), which is more in line with the theory of gravity, though not geology.
The conspiracy aspect supposedly relates to a large hole providing access to the underworld, which is located somewhere in the arctic or Antarctic regions, and the attempts to keep it quiet made by the superpowers in conjunction with whatever the Grand Theory du jour is, be it reptilians, Tibetan masters, Atlanteans and/or aliens. That there are undisclosed entrances to these "inner worlds" extends also to the moon, with one video purporting that you can see it on Google Earth, though it should be noted that this is very clearly just a graphics glitch caused by stitching multiple low-resolution and high-contrast images together in combination with the usual seam errors produced at the poles when using spherical UV mapping. These theories are also associated with apocryphal stories of apparently bottomless holes in various locations, such as Mel's Hole in Washington, USA.
It's not clear what supports the surface of the Earth according to the Hollow Earth Theory, since the surface of the earth we can see would most likely collapse under the force of gravity without a substrate (the mantle and inner core are solid, the outer core is liquid, and both states are highly resistant to compression). Nor is it clear what keeps the denizens from floating away from the inner surface.
There's also the problem that the "shell" would need to be really dense, in order to produce a gravitational field this strong.
Finally, there's the question of exactly why this would be kept secret - as with several other conspiracy theories, there seems to be no motive behind this one aside from "Let's make the unwashed masses look really stupid!"
 See also
- Cellular cosmogony (Earth as the inner surface of a hollow)
- Expanding Earth
- Flat Earth
- Richard Shaver
- Time Cube
- Ernst Zundel
- ↑ Gravity
is part of the conspiracydoesn't work this way; they'd be weightless.
- ↑ 
- ↑ Moon Pole Secret
- ↑ Gravity = Universal Gravitational constant * mass of object1 * mass of object 2 over the radius squared. The average density of earth's crust is 3g/cm3 and is ~10km thick. So by their reckoning (4/3)π*radius of the earth - (4/3)π*(radius of the earth -10km) is their volume of the earth (1215778930*4π/3km3). And the mantle and core are much denser than the crust. So that number * 3X1012 kg/km3 = 1.57978336*1022 kg. Or ~.2% of earth's mass...