The name first appears in 1873, in Louis Jacolliot's Les Fils de Dieu, where a Brahmin acquaintance in Chandernagore tells him of an ancient underground realm called "Asgartha." The idea might have stayed a minor footnote if Saint-Yves d'Alveydre had not taken it up thirteen years later in Mission de l'Inde en Europe — claiming he had learned of Agartha directly from a teacher named Hardjji Scharipf while studying Sanskrit, and that he had subsequently reached the kingdom himself through astral projection rather than travel.
Saint-Yves described Agartha as a vast network of underground cities beneath the Himalayas, home to millions of enlightened inhabitants living under a system he called Synarchy — a harmonious fusion of spiritual, scientific and political authority, governed by a Sovereign Pontiff and a supreme ruler known as the Brahâtma, "the King of the World." He presented it less as a fantastical kingdom than as a working model for how European civilisation itself might be reorganised.