Agartha is the Hindu/Buddhist/Tibetan underworld paradise, sortof, with Shamballa as it's capital city.
http://en.wikipedia.org/wiki/Agartha
The Agarthans themselves might be based on the Shaver mysteries, or Brad Steiger's article "The Hollow Earth, Myth or Reality" in 'Atlantis Rising'.
By the way, I use Wikipedia because it's often a good place to start this kind of research, but it should in no way be confused with ultimate wisdom on any given subject. It just makes it easier for me to point people in the right direction, on a specific subject.