While present-day philologists and classicists agree on the story's fictional character, there is still debate on what served as its inspiration. As for instance with the story of Gyges, Plato is known to have freely borrowed some of his allegories and metaphors from older traditions. This led a number of scholars to investigate possible inspiration of Atlantis from Egyptian records of the Thera eruption, the Sea Peoples invasion, or the Trojan War. Others have rejected this chain of tradition as implausible and insist that Plato created an entirely fictional nation as his example, drawing loose inspiration from contemporary events such as the failed Athenian invasion of Sicily in 415–413 BC or the destruction of Helike in 373 BC.
The only primary sources for Atlantis are Plato's dialogues Timaeus and Critias; all other mentions of the island are based on them. The dialogues claim to quote Solon, who visited Egypt between 590 and 580 BC; they state that he translated Egyptian records of Atlantis. Written in 360 BC, Plato introduced Atlantis in Timaeus:
For it is related in our records how once upon a time your State stayed the course of a mighty host, which, starting from a distant point in the Atlantic ocean, was insolently advancing to attack the whole of Europe, and Asia to boot. For the ocean there was at that time navigable; for in front of the mouth which you Greeks call, as you say, 'the pillars of Heracles,' there lay an island which was larger than Libya and Asia together; and it was possible for the travelers of that time to cross from it to the other islands, and from the islands to the whole of the continent over against them which encompasses that veritable ocean. For all that we have here, lying within the mouth of which we speak, is evidently a haven having a narrow entrance; but that yonder is a real ocean, and the land surrounding it may most rightly be called, in the fullest and truest sense, a continent. Now in this island of Atlantis there existed a confederation of kings, of great and marvelous power, which held sway over all the island, and over many other islands also and parts of the continent.
The four people appearing in those two dialogues are the politicians Critias and Hermocrates as well as the philosophers Socrates and Timaeus of Locri, although only Critias speaks of Atlantis. In his works Plato makes extensive use of the Socratic method in order to discuss contrary positions within the context of a supposition.
The Timaeus begins with an introduction, followed by an account of the creations and structure of the universe and ancient civilizations. In the introduction, Socrates muses about the perfect society, described in Plato's Republic (c. 380 BC), and wonders if he and his guests might recollect a story which exemplifies such a society. Critias mentions a tale he considered to be historical, that would make the perfect example, and he then follows by describing Atlantis as is recorded in the Critias. In his account, ancient Athens seems to represent the "perfect society" and Atlantis its opponent, representing the very antithesis of the "perfect" traits described in the Republic.
According to Critias, the Hellenic deities of old divided the land so that each deity might have their own lot; Poseidon was appropriately, and to his liking, bequeathed the island of Atlantis. The island was larger than Ancient Libya and Asia Minor combined, but it was later sunk by an earthquake and became an impassable mud shoal, inhibiting travel to any part of the ocean. Plato asserted that the Egyptians described Atlantis as an island consisting mostly of mountains in the northern portions and along the shore and encompassing a great plain in an oblong shape in the south "extending in one direction three thousand stadia [about 555 km; 345 mi], but across the center inland it was two thousand stadia [about 370 km; 230 mi]." Fifty stadia [9 km; 6 mi] from the coast was a mountain that was low on all sides ... broke it off all round about ... the central island itself was five stades in diameter [about 0.92 km; 0.57 mi].
In Plato's metaphorical tale, Poseidon fell in love with Cleito, the daughter of Evenor and Leucippe, who bore him five pairs of male twins. The eldest of these, Atlas, was made rightful king of the entire island and the ocean (called the Atlantic Ocean in his honor), and was given the mountain of his birth and the surrounding area as his fiefdom. Atlas's twin Gadeirus, or Eumelus in Greek, was given the extremity of the island toward the pillars of Hercules. The other four pairs of twins—Ampheres and Evaemon, Mneseus and Autochthon, Elasippus and Mestor, and Azaes and Diaprepes—were also given "rule over many men, and a large territory."
Poseidon carved the mountain where his love dwelt into a palace and enclosed it with three circular moats of increasing width, varying from one to three stadia and separated by rings of land proportional in size. The Atlanteans then built bridges northward from the mountain, making a route to the rest of the island. They dug a great canal to the sea, and alongside the bridges carved tunnels into the rings of rock so that ships could pass into the city around the mountain; they carved docks from the rock walls of the moats. Every passage to the city was guarded by gates and towers, and a wall surrounded each ring of the city. The walls were constructed of red, white, and black rock, quarried from the moats, and were covered with brass, tin, and the precious metal orichalcum, respectively.
According to Critias, 9,000 years before his lifetime a war took place between those outside the Pillars of Hercules at the Strait of Gibraltar and those who dwelt within them. The Atlanteans had conquered the parts of Libya within the Pillars of Hercules, as far as Egypt, and the European continent as far as Tyrrhenia, and had subjected its people to slavery. The Athenians led an alliance of resistors against the Atlantean empire, and as the alliance disintegrated, prevailed alone against the empire, liberating the occupied lands.
But afterwards there occurred violent earthquakes and floods; and in a single day and night of misfortune all your warlike men in a body sank into the earth, and the island of Atlantis in like manner disappeared in the depths of the sea. For which reason the sea in those parts is impassable and impenetrable, because there is a shoal of mud in the way; and this was caused by the subsidence of the island.
The logographerHellanicus of Lesbos wrote an earlier work entitled Atlantis, of which only a few fragments survive. Hellanicus' work appears to have been a genealogical one concerning the daughters of Atlas (Ἀτλαντὶς in Greek means "of Atlas"), but some authors have suggested a possible connection with Plato's island. John V. Luce notes that when Plato writes about the genealogy of Atlantis's kings, he writes in the same style as Hellanicus, suggesting a similarity between a fragment of Hellanicus's work and an account in the Critias. Rodney Castleden suggests that Plato may have borrowed his title from Hellanicus, who may have based his work on an earlier work about Atlantis.
Castleden has pointed out that Plato wrote of Atlantis in 359 BC, when he returned to Athens from Sicily. He notes a number of parallels between the physical organisation and fortifications of Syracuse and Plato's description of Atlantis. Gunnar Rudberg was the first who elaborated upon the idea that Plato's attempt to realize his political ideas in the city of Syracuse could have heavily inspired the Atlantis account.