"The Moon Pool" is a fantasy/science-fiction novel by Abraham Merritt. The plot concerns an advanced race which has developed within the Earth's core. Eventually their most intelligent members create an offspring. This created entity encompasses both great good and great evil, but it slowly turns away from its creators and towards evil. The entity is called either "the Dweller" or "the Shining One"... This is just scratching the surface of an incredibly complex and suspenseful plot.
While on a scientific expedition to the South Sea Islands, Dr. David Throckmartin discovers among ancient ruins the Moon Pool, a portal into Muria, an unknown underground world. After the disappearance of Throckmartin, his wife and two companions, his old friend Dr. Walter Goodwin enters Muria with a rescue party, only to confront a fantastic subterranean world filled with incredible beings, astounding scientific advances, and the worship of the most evil of all creatures, The Shining One.