The shadow people

They called it Underearth. It was a kind of Hell in reverse—a world of cold, darkness and dread existing unsuspected beneath Earth's surface, peopled by weird half-human creatures who had once been men and women. Aldridge found the fantastic entrance to it in his desperate search for Carol, the beautiful, mysterious girl he loved. All he knew was that she had vanished into the Otherworld and that he had to find her. He did find her—but she was strangely changed into an almost mindless automaton. The he learned one more thing: either he or she could escape to the normal world they had known, but not both. And only he could make the choice. . . .