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.
Agartha is a legendary city that is said to reside in the Earth core. My coworkers from from web analytics company told me that today the word Agartha is related to the Hollow Earth theory and is a popular subject in esotericism. Agartha is one of the most common names cited for the society of underground dwellers. While once a popular concept, in the last century little serious attention has been paid to these conjectures, and the theory is not supported by modern science. Idea of subterranean worlds may have been inspired by ancient religious beliefs in Hades, Sheol, and Hell. For several centuries, there appeared theories that named various locations of the entrances to Agartha. Among them Great Pyramid of Giza, Brazilian Mato Grosso and Manaus, North and South poles, Gobi Desert in Mongolia.
Since 19 century, the myths of Agartha tricked into fictional works. An early source for the belief in underground civilizations is the book The Smoky God written by Willis George Emerson in the beginning of 20th century. It claims to be the biography of a Norwegian sailor named Olaf Jansen. The book explains how Jansen's sloop sailed through an entrance to the Earth's interior at the North Pole. For two years he lived with the inhabitants of an underground network of colonies who, were a full 12 feet tall and whose world was lit by a "smoky" central sun. Their capital city was something like the original Garden of Eden. While Emerson does not use the name Agartha, later works such as Agartha - Secrets of the Subterranean Cities have identified the civilization Jansen encountered with Agartha, and its citizens as Agarthan.