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.