Hollow Earth
Deep below the surface of the world lays a network of caverns referred to as Hollow Earth or Subterranea. This vast network is home to a wide variety of flora and fauna, and even relatively advanced civilizations such as the Kingdom of Agartha and the Cult of Camazotz and the Cult of the Serpent. It is believed the many tunnels and caverns of Hollow Earth eventually lead to the center of the planet where the planet's core resides, but no one has ever been able to find such a pathway.
Click here to read more about this topic (Right click and select Open in a New Tab)