Ragnarok proposes that a comet impacted the Earth tens of thousands of years ago; the impact produced the 'Drift' layers which have been attributed to the Ice ages; this event destroyed a civilization which had high technology, a civilization which vanished completely except for some myths; the disaster was accompanied by catastrophic fire followed by years-long cloud cover and extreme cold. Humanity survived only by hiding in deep caves; when they re-emerged they had to restart civilization from scratch. Donnelly provides extensive geological, archeological, astronomical and mythological evidence for this theory.--Sacred Texts