"Ragnarok : the Age of Fire and Gravel" by Ignatius Donnelly is a companion to the more well-known work Atlantis: The Antediluvian World. Donnelly argues that an enormous comet hit the earth 12,000 years ago, resulting in widespread fires, floods, poisonous gases, and unusually vicious and prolonged winters. The catastrophe destroyed a more advanced civilization, forcing its terrified population to seek shelter in caves. As cave-dwellers, they lost all knowledge of art, literature, music, philosophy, and engineering.