"Everything in the world," said the Gelong, "is constantly in a state of change and transition--peoples science, religions, laws and customs. How many great empires and brilliant cultures have perished! And that alone which remains unchanged is Evil, the tool of Bad Spirits. More than sixty thousand years ago a Holyman disappeared with a whole tribe of people under the ground and never appeared again on the surface of the earth. Many people, however, have since visited this kingdom, Sakkia Mouni, Undur Gheghen, Paspa, Khan Baber and others. No one knows where this place is. One says Afghanistan, others India. All the people there are protected against Evil and crimes do not exist within its bournes. Science has there developed calmly and nothing is threatened with destruction. The subterranean people have reached the highest knowledge. Now it is a large kingdom, millions of men with the King of the World as their ruler. He knows all the forces of the world and reads all the souls of humankind and the great book of their destiny. Invisibly he rules eight hundred million men on the surface of the earth and they will accomplish his every order."

Prince Chultun Beyli added: "This kingdom is Agharti. It extends throughout all the subterranean passages of the whole world. I heard a learned Lama of China relating to Bogdo Khan that all the subterranean caves of America are inhabited by the ancient people who have disappeared underground. Traces of them are still found on the surface of the land. These subterranean peoples and spaces are governed by rulers owing allegiance to the King of the World. In it there is not much of the wonderful. You know that in the two greatest oceans of the east and the west there were formerly two continents. They disappeared under the water but their people went into the subterranean kingdom. In underground caves there exists a peculiar light which affords growth to the grains and vegetables and long life without disease to the people. There are many different peoples and many different tribes. An old Buddhist Brahman in Nepal was carrying out the will of the Gods in making a visit to the ancient kingdom of Jenghiz,--Siam,--where he met a fisherman who ordered him to take a place in his boat and sail with him upon the sea. On the third day they reached an island where he met a people having two tongues which could speak separately in different languages. They showed to him peculiar, unfamiliar animals, tortoises with sixteen feet and one eye, huge snakes with a very tasty flesh and birds with teeth which caught fish for their masters in the sea. These people told him that they had come up out of the subterranean kingdom and described to him certain parts of the underground country."

Originally posted by Skyfloating
The place is sometimes seen as a physical location but has also been mythologized to be the Buddhist version of Heaven (called "Pure Land"), full of enlightened inhabitants who have accumulated positive karma and are therefore permitted entry.