Of course the only thing that perked my interest was "Mammoth Cave" around the 10 O'clock position on the circle. I've been to Mammoth Cave several times and nobody is allowed to explore the cave on their own. They only allow you to go so many hundreds of feet into the cave, then up the stairs you go out to the surface. I often wondered how far back the cave winds down.
I've heard of the Hollow earth theory before. Not saying I accept it or reject it. There's just not enough evidence for me to make base my opinion on. But of course, I don't research it either.