Here: http://www.crystalinks.com/hollowearth.html. 
Now, I was never saying that the Earth "is" hollow, I said it "can" be. There is a possibility, even though it is not likely. It is an interesting theory that has been around for a long time, based off of several people who have claimed to either been down in the "Hollow Earth" or seen the apparent hole in the North Pole. Furthermore, I do not believe that the Earth is flat, it has been scientifically proven several times. However, this "Hollow Earth" theory has not been scientifically proven or debunked, thus it is still a mystery.