Agartha (also spelled Agartta, Agharti, Agarath, Agarta, Agharta, or Agarttha) is a legendary kingdom that is said to be located on the inner surface of the Earth. It is sometimes related to the belief in a hollow Earth and is a popular subject in esotericism, occultism, and the New Age. The concept was introduced by Louis Jacolliot in his 1873 book Les Fils du Dieu, and was expanded upon by authors Alexandre Saint-Yves d'Alveydre, Ferdynand Ossendowski and René Guénon. Saint-Yves's version of the tale would become the most influential.