Critias, a dialogue of Plato's, speaks about a variety of subjects. In it, Plato mentions that 9,000 years previously, the Athenians, due to their ordered society had been successful in defeating the mighty empire of Atlantis. Atlantis was a large continent sized body of land "larger than Asia and Africa combined" which was located near the Straits of Gibraltar. Only part of the dialogue is known to us today – the latter portion has been lost.
Critias follows the dialogue of Timaeus, also written by Plato, a dialogue between the same men: Socrates, Critias, and Timaeus. Another Critias (great grandfather of the Critias in the dialgue) and Solon are also referred to in the text. All of these men are known to have existed, except Timaeus, of which no independent historical record exists.
According to Critias, in ancient times, the Earth was divided among the gods by allotment. The gods treated the humans in their districts much as shepherds treat sheep -- tending and guiding them like nurselings and possessions. They did this not by force, but by persuasion.
In those days, the areas which are now the islands of Greece were high hills covered in good soil. But a number of great deluges came (including the global flood of Deucalion), and because no soil washed down from the mountains to replace the lost soil, the soil in that land was stripped away, causing much of the area to sink out of sight, and the islands that remained to become the "bones of a dead body."
Athens in those days, however, was very different. The land was rich and water was brought in from underground springs (which were later destroyed by earthquake). He describes the civilization of Athens at that time as ideal: pursuing all virtue, living in moderation, and excelling in their work.
He then moves on to describe the origins of Atlantis. He said that Atlantis was allotted to Poseidon. Poseidon fell in love with a mortal girl named Cleito, and she bore him a number of children. Poseidon's first-born son was Atlas, who inherited the kingdom, and passed it onto his firstborn for many generations.
Critias then reiterates the remarkable virtue of the Atlanteans, saying:
However, he then recounts that over time the Atlanteans became corrupt as:
Critias then says that Zeus, the god of gods, seeing the corruption of the Atlanteans, determined to chastise them. Zeus begins to speak; but what he says, and everything that follows in the Critias, has been lost.