[5] In 1923, along with University of Michigan botanist Carl D. LaRue and plant pathologist James Weir, Jessup participated as a photographer in a U.S. Department of Agriculture expedition to Brazil to study the possibility of growing rubber in the Amazon.
"[8] Jessup also "linked ancient monuments with prehistoric superscience";[9] years later similar claims were made by Erich von Däniken in Chariots of the Gods?
A copy of The Case for the UFO unwittingly became the nexus of a whole other conspiracy theory when Carl Meredith Allen (sometimes calling himself Carlos Miguel Allende), an ex-merchant marine, sent it covered in scribbled notes to a US Navy research institute, and sent a series of letters to Jessup himself, laying out an incident Allen claimed to have witnessed during World War II where the US Navy made a ship invisible and accidentally teleported it through space, the so called "Philadelphia Experiment".
Jessup's main flying-saucer scenario came to resemble that of the Shaver Hoax perpetrated by the science-fiction magazine editor Raymond A. Palmer—namely, that "good" and "bad" groups of space aliens were/are meddling with terrestrial affairs.
Like most of the writers on flying saucers and the so-called contactees that emerged during the 1950s, Jessup displayed familiarity with the alternative mythology of human prehistory developed by Helena P. Blavatsky's cult of Theosophy, which included the mythical lost continents of Atlantis, Mu, and Lemuria.
[15] Friends interviewed in the same book thought the bizarre letters from Carl Meredith Allen may have initiated a decline in Jessup's mental state leading to his eventual suicide.