Agartha, the Hollow Earth

Agartha is a pocket dimension that simultaneously exists inside the Astral plane, and inside a series of caverns and caves located deep within the Earth. It is a "lost world," filled with ancient civilizations that never abandoned magic, and creatures from out of time, such as smilodons and tyrannosaurs. Agartha can be reached by following certain caverns down beneath the Earth, but it cannot be picked up with conventional seismic detectors because it exists partially out-of-phase with our reality.