Hollow Earth Theory
Is the Earth solid all the way down to its core or are there places under the Earth capable of supporting life?
Some people believe that it is possible and it is called the Hollow Earth Theory. I have no idea if the theory is true or not and I really do not care. It does, however, make for some interesting storytelling. Image yourself walking home, taking a short cut through the woods, and falling into a hole. You do not fall to your death; instead, you end up in another civilization. The natives arrest you, thinking that you are a spy, and hold you prisoner with the intent of executing you in three weeks. In the meantime, you befriend a beautiful native girl and you escape. At the path, which leads you back to your own world, an arrow hits the beautiful native girl, killing her graveyard dead. It was a good thing that she was there or the arrow would have been in your back. You make it back to your home, but no one believes your story.
Finally, you crack up and end up in an insane asylum.