What is that? It’s crazy? Well maybe, but that’s all a matter of opinion. Crazy or not however, the Hollow Earth theory makes for some fun reading. There are some aspects of it that are just as imagined as any fantasy world (Note: subterranean civilizations can be common in fantasy and sci-fi) created by authors over the last century or so. Also, what’s not entertaining about a theory that the Earth is hollow and filled with either, green skinned women, Nazis, reptoids, aliens, ape men, prehistoric throwbacks, lost civilizations or variations there of? It’s just simply fun. It’s like reading the pulp fiction stories of old, but with the very slim chances that some of it might be real.
Seeing as there is a wealth of concepts and theories surrounding the Hollow Earth theories, I'm really only going to stick with a couple of the most common beliefs and theories surrounding it.
First off we have the connection to the famous explorer Admiral Byrd. Supposedly he wrote a diary during his expeditions to the Antarctic in which it is claimed that he went to the so called Hollow Earth. This theory is only really toted by author and publicist Raymond W. Bernard, although some people still follow it today. Raymond apparently took so fully to the Hollow Earth theory however, that it killed him (At least he died doing something he enjoyed, always a plus!) while searching for entrances to it in South America. He was also one of the first people to claim that UFOs come from the world below.
Next we have the Nazi connection, which really shouldn’t come as a surprise to anyone; those pesky Nazis seem to have been into everything. Anyways, as theory goes, the Germans had discovered an entrance to the Hollow Earth in Antarctic waters, which they promptly escaped to at the end of WWII, with or with out Hitler. According to some groups they live on there till this day plotting to take over the world. Although I believe they probably went there for the green women. After all, Nazis have to have fun too and of course after a loss like theirs, they were probably in need of something to lighten the mood.
For the last bit, I bring in the reptoids. Yep, them scaly buggers from one of my earlier posts, who are also purported to live in the Earth’s hollows, whether it’s the fully hollow version, or the large cavern pocketed one. The main theory here is that they are either dinosaur descendants that survived in subterranean dwellings or they came from space. In the end they pretty control everything on Earth through various means, but dwell mainly below ground. Whatever though, it’s just more strangeness to add to the Hollow Earth theories floating around and is no more probable then the Nazis frolicking with Admiral Byrd and them delightful green skinned women. Oh, and don’t bother asking me anything about the green skinned women, I can’t remember how in the world they’re involved in this Hollow Earth shindig. There’s not much info on that bit, but hey, they make great comedic material don’t they?

Sources & Further Reading:
Hollow Earth - Wikipedia
Hollow Earth, Subterranean Civilizations, Agartha - Crystalinks.com
Our Earth is Hollow!
The Hollow Earth - Fact or Fiction?
Hollow Earth - Tinwiki