Hollow Earth Theory
I've always been fascinated by the idea that there is another civilization living beneath our feet. The theory has been around since ancient times - or at least ever since we realized the earth isn't flat - and it's stuck around in one form or another since. I don't know if it's ever reached widespread acceptance at any point in it's history, but it's always had a reasonably-sized following.
There are a few different models of how our earth could be hollow. One of the older, models, which isn't quite as "mainstream" (haha) now, is the concentric shell theory. It states that the Earth isn't exactly hollow, but it has other hollow earths inside of it! Apparently, the mathematician that came up with it said there were exactly three earths within our earth. I'm sure you're wondering - wouldn't it be dark in there? Other models depend on a central sun, what's going on in this model to keep it from just being a huge cave? Well scientists believed that the atmosphere within the earth was luminous! It didn't need a sun, EVERYTHING within the earth glowed like daylight! The most interesting part of this theory is that the northern lights, the amazing light patterns shown at dawn and dusk in Alaska, Northern Canada, and other regions close to the north pole, was explained as escaping luminous atmosphere from within the earth! It think this explanation makes a lot of sense, and is at least easier to understand than science's current explanation about magnetism or something.
I like this explanation because it just makes sense! It's so easy for me to conceptualize. It's really hard for me to accept the idea of a separate sun being at the center of our hollow earth. I think some hollow earth theories involve massive openings at the north and south poles, through which OUR sun enters to light up the center of the hollow earth when it's night time on the surface. Now I'm all for a good conspiracy, and I can chalk up a certain amount of contrary evidence to a government conspiracy, but I'm pretty certain folks in China have light when it's dark over here. This hollow earth model might have made more sense back in the day, when we didn't know anything about the world as a whole, but now it seems pretty ridiculous. I think the most common theory now involves a small sun at the center of the planet. Now that I'm thinking about it, it does kinda make sense. The small sun would provide the same heat at the center of our Earth that scientists ascribe to the molten core.
If you want to read more about this stuff, I would HIGHLY recommend checking out Conspiracy Critic's page on the Hollow Earth theory. I love their site - it's like a snopes.com or conspiracists. That site goes into some of the other models and has a section on proposed inhabitants within the hollow earth that I thought was amazing. I even verified with other sites - Hitler believed in a hollow earth, and a lot of people believe he found the opening and escaped! He could be living there today! They never found his body you know....