Shaver's stories continued to appear in Amazing after Howard Browne replaced Ray Palmer as editor.
Even after his work fell out of favor with Amazing readers, Ray Palmer continued to publish Shaver in other genre magazines.
A special issue of Fantastic devoted to the "Shaver Mystery" was published in 1958
He achieved notoriety in the years following World War II as the author of controversial stories that were printed in science fiction magazines (primarily Amazing Stories), in which he claimed that he had had personal experience of a sinister, ancient civilization that harbored fantastic technology in caverns under the earth. The controversy stemmed from the claim by Shaver, and his editor and publisher Ray Palmer, that Shaver's writings, while presented in the guise of fiction, were fundamentally true. Shaver's stories were promoted by Ray Palmer as "The Shaver Mystery".
During the last decades of his life, Shaver devoted himself to "rock books"—stones that he believed had been created by the advanced ancient races and embedded with legible pictures and texts. He produced paintings based on the rock images and photographed the rock books extensively, as well as writing about them. Posthumously, Shaver has gained a reputation as an artist and his paintings and photos have been exhibited in Los Angeles, New York and elsewhere.
Shaver claimed to have worked in a factory where, in 1932, odd things began to occur. As Bruce Lanier Wright notes, Shaver "began to notice that one of the welding guns on his job site, 'by some freak of its coil's field atunements', was allowing him to hear the thoughts of the men working around him. More frighteningly, he then received the telepathic record of a torture session conducted by malign entities in caverns deep within the earth". According to Michael Barkun, Shaver offered inconsistent accounts of how he first learned of the hidden cavern world, but that the assembly line story was the "most common version". Shaver said he then quit his job, and became a hobo for a while.
Barkun writes that "Shaver was hospitalized briefly for psychiatric problems in 1934, but there does not appear to have been a clear diagnosis". Barkun notes that afterwards, Shaver's whereabouts and actions cannot be reliably traced until the early 1940s. In 1971, Ray Palmer reported that "Shaver had spent eight years not in the Cavern World, but in a mental institution".