This companion book to Donnelly's Atlantis, the Antediluvian World, published one year later, is less well known today. Ragnarok is out of print except through specialty print-on-demand publishers, while Atlantis, the Antediluvian World is in print and fairly easy to obtain. Ragnarok, The Age of Fire and Gravel, proposes that a comet impacted the Earth several tens of thousands of years ago; the impact produced the 'Drift' layers of gravel 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. The book is not academic and often sensationalistic, but his populist style does not seem to detract from the argument.
Today, mass extinction from cometary impact is considered mainstream science, supported by a huge body of physical evidence. In Donnelly's time it was unknown that cometary or asteroid impacts had even occurred on this planet. There was very little knowledge of the structure or nature of comets. Today, we have sent probes into Halleys' comet, landed a probe on an asteroid, and witnessed the impact of Comet Shoemaker-Levy on Jupiter. We also have extensive data about impact craters on the Earth, Moon, Mars and other planets and moons. The impact event which wiped out the dinosaurs has been validated by a layer of iridium, an otherwise rare element, which appears in the strata in the layer just above the dinosaurs, and a recently discovered impact crater in the Yucatan. There have also been other mass extinctions in the geological past, for instance the Permian extinction before the age of the dinosaurs which also wiped out nearly all life on earth. This growing knowledge of the risk of impact events to our civilization was popularized recently in a pair of big-budget disaster movies: Deep Impact