In 1974, he published The Riddle of the Pyramids, in which he sought to explain the whys and wherefores of the earliest Egyptian pyramids. Though Mendelssohn himself was not an Egyptologist, the book builds on advice from experts like Sir Robert Mond and Walter Emery, as well as his own visits to Egypt and Mexico. His principal thesis was that the pyramid at Meidum had collapsed during construction, a conclusion he arrived at using his knowledge of physics and which was sparked in 1966 by images of the Aberfan disaster, where Mendelssohn saw similarities to the rubble mound surrounding the Meidum pyramid, a primary destination for his travel to Egypt the year before. Working from that conclusion, he further elaborated a theory that pyramid construction in Egypt took on a life of its own during the Third and Fourth Dynasties, more or less independently of the reigns of pharaohs. His theory has not been taken up by the Egyptological community, but the book remains a stimulating and detailed study of the Egyptian pyramids.