Giants of Agartha
The Giants of Agartha are said to be a race of subterranean giants living in a network of cities spanning the entire length of the globe – albeit completely underground. They are thought to be 12-feet tall and live in the center of the planet, where a “smoky” sun keeps things lit and warm.
Inner Earth Aliens
Apparently, US Navy Vice Admiral Richard Byrd is said to have discovered an entrance to an alien civilization hidden away at the North Pole. A massive fissure in the ice is said to have opened up this subterranean world.