Although I'm not a proponent of Hollow Earth theory, I believe it is worth mentioning the (pseudo?)scientific concept behind this theory. Many people on this forum may have heard of Hollow Earth theory but I doubt many of them can explain what this theory actually claims from a physics point of view.
Hollow Earth theory states that the earth's center of gravity is just a few hundred kilometers below us. According to this theory, the earth's gravity is the consequence of the electro-magnetic features of this center of gravity (functioning much like an electromagnetic coil spool). The center of the earth is (so they say) Hollow and people walk on oposite side of us, as they are on the other side of the center of gravity.
In the middle is a central sun. According to Hollow Earth theory, all planets originated as gas orbs (must like a star) that cooled down until the outer parts became solid. Because of centrifugional powers, there are holes at both poles allowing entry into the Hollow Earth and back.
Proponents of Hollow Earth theory consider Aurora Borealis to be caused by rays from the inner sun shining through the polar openings and affected by the electro-magnetic fields it passes through.
It is said that the origin of human culture exists within Hollow Earth. It is said that a few remaining Nazis moved to Antarctica and entered the Hollow Earth opening to rebuild their society safely. It is said that admiral Byrd once flew into one of the polar openings and wrote a report of his findings.
The major problem with this theory (except for the unconventional physics) is the fact that not a single picture, artifact or person from within Hollow Earth has surfaced. If there really were these openings, certainly at least someone would have entered Hollow Earth and taken some proof with him, right?! At least someone would have exposed that proof and put it on the Internet, right? Certainly there are a few people who claim to be organising a (very expensive) trip to inside Hollow Earth for several years, however to this day Hollow Earth theory seems nothing more than an unsubstanciated hypothesis.
Another big problem with this theory, is that is requires the complicity of NASA as well as many governments considering you don't just hide two huge holes in the earth even if they're at the coldest places. Some say the holes clearly show or were obviously photoshopped out in some of the NASA footage, however these claims are dubious to say the least.