The King of the Mole Men mentions three underground kingdoms: Pellucidar, Agartha, and Symzonia. Pellucidar is a Hollow Earth created by Edgar Rice Burroughs, writer of the Tarzan stories. Agartha is a legendary city in the earth's core. Symzonia is a fictional land at the center of the earth, believed to have been invented by John Symmes, who believed that the earth is hollow.