kingdom of Agartha ended up as one of the most viewed of my blog — which confirms the general fascination with "Hollow Earth"-themed fiction.
I have found further information about Agartha on the excellent Penny Dreadful wiki:
Agartha is an ancient kingdom in either Sri Lanka or Tibet (travellers are not sure which, for reasons which are explained below). The kingdom, which may be mythical, would seem to have a strange effect on outsiders: they either do not notice it as they travel through it, or they forget about it once they have seen it. There are many rumours about Agartha, however. It is said that its capital, Paradesa, holds the University of Knowledge, where the occult and spiritual treasures of mankind are guarded. The capital also is home to an enormous gilded throne which is said to be decorated with the figures of two million gods, and it is further rumoured that their combined good spirits are what hold the world together; if they are angered by a mortal, their wrath will descend upon the world, drying the seas and smashing the mountains into deserts. Finally, it is said that Agartha holds the world's largest library of stone books, and that strange fauna inhabit the kingdom, including sharp-toothed birds and six-footed turtles as well as the natives, who are born with forked tongues. The guardians of Agartha are the Templars of Agartha, a small but powerful army.