Physics of the Hollow Earth: Strange Reality

While man has spared no expense to explore the heavens, we know precious little about what's right beneath our feet. The beginnings to unlocking that mystery start with the discussion of whether the Earth is solid or hollow. If it's hollow, as many physicists believe, that fact has profound implications for our future. The key to endless energy may be in that void in the center of the planet. Lost civilizations may be down there as well as the literal heavens and hells of our great religions. Is there a vast, secret, long-forgotten cave system through the Earth? Could this labyrinth still be in use, or capable of use? What exactly is down there?