Atlantis was the name of a mysterious city in many mythologies originating from Earth and was, specifically, according to Andrew Ketterley, a very real place that was the first great civilisation on Earth, existing near or at the very beginning of the world.
Atlantean society was so advanced (in such a primitive era) that its people had even learned to travel to others worlds. In fact, seeing as Atlantean society seemed to have disappeared from Earth, all of its citizens may have potentially physically moved into another world, leaving Earth behind and taking with them the secrets of their advanced understanding of both the natural and supernatural universe. Another plausible theory is that Atlanteans were not native to Earth, but had merely visited the planet for a short time during their journey to many other worlds. Andrew Ketterley described a connection between fairy people living on Earth and Atlantis, which seemed to suggest that the people of Atlantis might not have been humans but, perhaps, more magic-sensitive beings like fairies.
When Polly Plummer vanished out of our world into the Wood Between the Worlds, Andrew told his alarmed nephew, Digory Kirke, the story of how he had become able to transport people to other worlds. He described how his "fairy" god mother, Mrs. Lefay (she was a human but supposedly with some fairy blood) had given him a box on her deathbed, telling him not to open it but to destroy it immediately. Knowing that it was ancient, but not from any primitive civilisation he could recognise, he did some research and confirmed it to have been created in Atlantis. Opening it, he discovered magic dust that had been brought from another world when our world was only just beginning, possibly suggesting that Atlantis existed on Earth even before humans as a species had developed. From this dust, Andrew succeeded in making the magic rings, which could take people from various worlds to the "Wood between the Worlds" and back again. It thus may have been that Atlantis forged links between our world and others, such as that mentioned in the claim by Mrs. Beaver that Jadis, a native of the world of Charn, was descended from Adam's first wifeLilith, a believed inhabitant of Earth.
- Atlantis as mentioned in The Magician's Nephew may be a reference to the island of Númenor in the mythology of J.R.R. Tolkien, a close friend of C.S. Lewis. The Legend of Númenor is a reshaping of the Greek myth of Atlantis, and Númenor is mentioned (albeit spelled 'Numinor') in 'That Hideous Strength', the final book of Lewis' less well known Cosmic Trilogy.