The Lost Continent: Atlantis:

The legend of originated from the Greek philosopher Plato around 360 BCE in his dialogues Timaeus and Critias . He described it as a powerful, technologically advanced naval empire located "beyond the Pillars of Hercules" (the modern-day Strait of Gibraltar) that sank into the ocean in a single day and night of catastrophic earthquakes and floods. The Origins: Plato’s Allegory

Most modern scholars believe Plato invented Atlantis as a to illustrate the dangers of hubris and moral decay. In his account, the once-noble Atlanteans became greedy and attempted to conquer Athens, only to be defeated and then punished by the gods. Plato likely used this story as a "thought experiment" to compare his version of an ideal city-state (Athens) against a corrupt, expansionist power. Atlantis: The Lost Continent