Atlantis (known by the title "The Ancient Fortress of Atlantis") was an island and fortress that existed in the Atlantic Ocean that one day sunk. In the anime, it is seen that the city sunk most likely due to to a violent storm.
- Atlantis is based on the island described by classic Athens philosopher Plato in his works Timaeus and Critias. In these texts, Atlantis serves as an allegorical representation of the hubris of nations, serving as Plato's counter-example for the ideal state he describes in his political texts such as The Republic. In Plato's texts, as well as in many later texts in literature and popular culture, the story of Atlantis concludes with the island falling out of favor with the gods and submerging into the Atlantic Ocean.
- As described by Franken Stein in the manga, Atlantis is known as a sort of "horror," as opposed to a utopia in the making. Both interpretations of Atlantis, as utopia and dystopia, have weight based on historical literary accounts and representations in more recent popular culture, however. For example, in Plato's text Critias, Atlantis serves to represent the antithesis to that author's ideal society, a community whose engagement in slavery and war led to its downfall. In some interpretations, Plato's Atlantis is treated as an allegory warning against an overzealous desire for conquest.