Shaver's stories—and all they claimed to explain—were promoted by Palmer as "The Shaver Mystery." Shaver wrote of tremendously advanced pre-historic races who had built cavern cities inside the earth before abandoning our earth for another planet. Those ancients also abandoned some of their own diseased offspring here, who degenerated over time into a population of mentally impared sadists known as Dero. These Dero still lived in the cave cities, according to Shaver, kidnapping surface-dwelling people for meat and using the fantastic "ray" machines that the great ancient races left behind to project tormenting thoughts and voices into our minds. Shaver claimed first-hand knowledge of the Dero and their caves.
Between 1945 and 1949, letters poured in attesting to the truth of Shaver's claims (tens of thousands of letters, according to Palmer): the correspondents, too, had heard strange voices or encountered denizens of the hollow earth. "Shaver Mystery Club" chapters sprang up in several cities. Many in the community of science fiction fans felt compelled to publicly condemn the Shaver Mystery as "the Shaver Hoax." The controversy gained some notice in the mainstream press at the time, including a mention in Life magazine.
In the 1960s and 70s, now living in obscurity, Shaver looked for physical evidence of the bygone pre-historic races. He found it in certain rocks, which he believed were "rock books" that had been created by the great ancients, and embedded with legible pictures and texts. For years he wrote about the rock books, photographed them, and made paintings of the images he found in them to demonstrate their historic importance. But Shaver's initial audience had moved on decades before, and he never succeeded in generating much attention for his later findings.