Deep beneath the earth's crust dwells a mysterious empire called Agharta. An entirely self-sufficient world situated in a massive cavern below the earth's outer shell, it possesses its own source of light, as well as abundant water and food. For thousands of years, its millions of inhabitants have evolved, developing technologies far superior to ours. A network of tunnels connect the cities of Agharta to many secret locations on the surface. Located at the very center of the earth, the capital city, Shamballah, is the home to the sovereign leader of this civilization, known as The King Of the World.
The denizens of Agharta are believed to represent the lineage of the Lost World of Atlantis, their ancestors having escaped underground from the destruction brought upon themselves through senseless nuclear conflict.
Some believe that flying saucers are not extraterrestrial, but are actually the aircraft of the Aghartans, who journey to the surface to observe our world with the purpose of preventing the human race from its own willful self-destruction.
Though the name Agharta is said to be of Buddhist origin, legends of the subterranean world are found in many different cultures, notably among the Native Americans of north and south Americas.
Sources:
www.ufoarea.com/hollow_agharta.html
www.livinginthelightms.com/in_search_of_shamballah2.html
see also:
The Hollow Earth by Dr. R. W. Barnard