Synopsis: From the back cover of the Ace first edition: "In the bleak and polluted world of the twenty-first century, a cure can mutate into something lethal and the toxic can become beneficial... Thus, the radon gas seeping from the abandoned uranium mine beneath the Planetary Health Organization's Rejuvenation Facility has been found to ease the pain of those stricken with all manner of bone diseases. But when one of the patients is found dead of extremely unnatural causes, District Marshal Ty Merrick has reason to question exactly how benign the deadly gas has become. Do the supposed killers - alien beings who live in the depths of the mine - actually exist, or are they radon-induced hallucinations? And if they don't exist, what exactly is going on down below. Now Merrick and partner LaRue must figure out what's real, what's not - and what's waiting for them when the cross the line..."