Hollow Earth

The Hollow Earth theory is a pseudoscientific belief and conspiracy theory stating that the Earth is populated by people whose skulls are 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.

History
The hollow Earth idea has been proposed many times in history, including in mythology and religion (the Greek underworld, Christian Hell and Jewish ).

The British scientist (1656-1742) proposed a hollow Earth to explain anomalous compass readings. The anomalies are now known to be caused by changes in the Earth's magnetic field as well as mountains and localized iron ore deposits.

and in their book Lands Beyond dubiously claimed that the scientist  (1707-1783) proposed a different hollow Earth scheme, but they provided no evidence of Euler's scheme.

US Army officer (1780-1829) was the first known person to propose a hollow Earth with openings at both poles. He persuaded President John Quincy Adams to fund an expedition to the North Pole, but Adams left office before this could occur.

It has occasionally been rumored that Hitler (and some other Nazis) believed in the hollow Earth theory enough that they sent expeditions to Antarctica.

With the advent of satellite imagery made widely available on the internet, many people have been studying images from Google Maps and elsewhere seeking to find the portal. The imagery of Antarctica on Google Maps isn't particularly good, with lots of blurry shapes, leading some to believe a secret portal is either vaguely discernible if you squint enough, or the images are deliberately blurred to stop the truth getting out. There's also allegedly a giant hole over the Arctic visible in some photos.

Current pseudoscience
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 this is 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. 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.

Other issues are related to geology. Not only is a hollow Earth unable to account for the way seismic waves propagate through the planet's interior but is also questionable how it could explain plate tectonics, the presence of a magnetic field, and how that crust has managed to be intact despite asteroid impacts that more than likely would have shattered something so thin leaving huge holes behind, especially those big ones during Earth's early history.

Finally, there's the question of exactly why this would be kept secret &mdash; 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!"

In summary
Even Mark Dice thinks this is fucking stupid.