Not one but a series of versions of the theory of the existence of a Hollow Earth. Superficially similar to the types of realms seen in the Middle Umbra, the Hollow Worlds are noteworthy because they can be reached by mundane humans through use of certain subterranean passages, mine shafts, and unremarkable elevators. Furthermore, some argue that the Hollow Worlds constitute an entirely different type of Umbra, and call it the "Inner Umbra", claiming that if Earth is a theatre and the world beyond the lobby is the Umbra, the area backstage is where the Hollow Worlds can be found. In any case attempts to cross the Gauntlet into the Hollow Worlds invariably fail, baffling most theorists on Umbral topography. The Sons of Ether are especially fond of this zone, because of how well suited it is to high adventure.