Agartha refers to a realm beneath the surface of the world. The concept of the area stems from the Hollow Earth theory, which states that the planet is not solid; rather it has a whole, mostly unexplored, world inside of it. Agartha will be used to assist players in traveling to certain destinations.

Since the beginning of the ARG, there have been hints that there is a world beneath our own. The Poem was transcribed with the words "Amundsen found it under the ice." Since then, along with The Photo and At the Door events, the community gathered that a world exists below the very one they inhabit, and several gateways to that realm can be found around the world.

Aveline Belmont tasked the community with helping her get through one doorway into Agartha in search of clues to her great-grandfather's past. The Sanctuary of Secrets was directly affected by Aveline opening the entrance.

Agartha, "a place of mystery, of discovery," contains anctient living machines in addition to the portals players use for fast, global travel.