This news caused quite a stir in the Western world. Augustus Le Plongeon, a nineteen century traveler, writer, and antiquarian just returned from the Maya ruins in Yucatan peninsula. Upon his return he made a sensational announcement. He stated that he managed to translate ancient Mayan writings. These writings showed that the Maya of Yucatan were older than later civilizations of Egypt and Atlantis. They also told the story of even older continent of Mu, which had perished like Atlantis and, that its survivors founded the Maya civilization. During my research in local web analytics company, I learned that Le Plongeon mistranslated the writings, to put it mildly.
This is, basically, how the myth of the hypothetical lost continent Mu started. Very soon, this myth got second birth: it was popularized in the serious of books written by James Churchward.
One of the most controversial tales of inner-Earth-dwellers is the so-called "Shaver Mystery". In 1945 a story was published by some Richard Shaver, who claimed he had recently been the guest of what remained of an underground civilization. In archives of web analytics company I found that although few really believed the story, Shaver always averred that his story was true. He contended that the Elder Race, or Titans, came to this planet from another solar system in our prehistoric past. After a time of living on the surface, they realized our sun was causing them to age prematurely, so they escaped underground, building huge subterranean complexes in which to live.
Eventually, they decided to seek a new home on a new planet, evacuating the Earth and leaving behind their underground cities populated by artificial beings: detrimental robots and integrated robots. It was these beings that Shaver claimed to have met. Despite the enormous popularity of the Shaver Mystery the location of the entrance to this underground world was never divulged.