This publication is devoted to an extensive description of the pneumopteria which are also referred to as cloud whales or cloud sponges, and occasionally, in scientific language, as pneumospongia. Often, in older treatises, they have been termed celestial leviathans. These gigantic, cloud-like creatures, which seemed to float in the air without movement or stimulation, could reach several hundred meters in size. In all likelihood, they are extinct today. The species was, however, frequently observed in historical times and has been described by numerous cultures and in many different contexts. With his research findings, Roland Boden provides the first thorough introduction to the nature, appearance, and behavior of the pneumopteria and surveys the history of their exploration and description. In his treatise, defined as fictional research, Boden uses quotations, photographs, reproductions, reconstructive drawings, and computer-generated images, combining real scientific and historical facts with completely fictitious elements to create a parallel reality that cleverly and artistically questions contemporary perceptual processes.