Examining a vast corpus of literary references and artistic representations, this volume offers a comprehensive study of the ketos - the type of sea monster imagined by the ancient Greeks and Romans. The chapters explore the three central traditions of thought that existed about this creature in Graeco-Roman culture. The first tradition concerns the ketos as a divinely associated monster: a force aligned with marine gods (chiefly Poseidon) and one which was fought by Heracles and Perseus. The second tradition features the ketos in a more naturalised context, as depicted among ancient geographers, as a type of monster roaming the distant waters of the Atlantic and Indian Oceans. The third tradition concerns the fusion of the ketos with the Old Testament sea monsters in the minds of early Christians. Accordingly, this classical sea monster became the image of the creature that swallowed Jonah, and, alternately, a monster associated with the devil. While other monsters of Graeco-Roman mythology, such as the Minotaur and Medusa, are household names in modern popular culture, the ketos is not as well remembered. Yet it was no small part of the Graeco-Roman imagination. This sea monster formed a key aspect as to how the sea-adjacent societies of ancient Greece and Rome perceived ancient marine environments. It was this fantastic sea beast that so haunted ancient mariners, and in turn, which contributed to ancient perceptions of the marine world as a profoundly alien and hostile environment.