Here we cover the main facts as given by Plato.
The Island of Atlantis was located "before the pillars of Hercules." The pillars of Hercules were the straits of Gilbralter.
Atlantis, according to Plato, sank in "a single day and night of misfortune".
For the alledged source of the story here is the version from History of Atlantis by Lewis Spence:
Critias: Listen then, Socrates, to a tale which, strange though it be, is yet perfectly true, as Solon, the wisest of the seven once said. He was a relation and friend of Dropidas, my great-grandfather, as he tells us himself in his poems, and Dropidas assured my grandfather, Critias, who, when an old man, repeated it to us, that there were great and marvellous exploits achieved by Athens in the days of old, which, through lapse of time and in the course of generations, have vanished from memory.
Notice that in the above Plato ascribes this "tale" to a revered ancestor and asserts it is "perfectly true". In multiple places Plato has the characters assert that the tale is true.
Roughly this gives a timeframe for the sinking of about 9,600 B.C. Under some analysis one can argue that the Timaeus gives a time for the sinking of after 8570 B.C. while the Critias gives a time of approximately 9421 B.C.
Plato's estimate for the size of Atlantis seems internally inconsistent. At one point he describes it as "larger than Libya and Asia". Libya here refers to the Mediteranean coast of North Africa. Asia probably refers to Asia Minor or roughly the Middle East. This makes Atlantis continent sized.
In contrast, Plato also says the center area of the city was "oblong shape, extending in one direction three thousand stadia, but across the centre inland it was two thousand stadia". Since a "stadium" is a unit of measure that is about 11 miles, this makes the "flat" area of Atlantis 330 miles by 220 miles. That is much less than a continent. Also some other descriptions in the dialogues make Atlantis seem still smaller. Interestingly, some Greek words within the dialogue also suggest quite different sizes for the land mass.
As for its topography, Critias says "The whole country was said by him to be very lofty and precipitous on the side of the sea."
Plato portrays the Atlanteans as having a fine character. However, before their submergence their character took a turn for the worse.
For many generations, as long as the divine nature lasted in them, they were obedient to the laws, and well-affectioned towards the god, whose seed they were; for they possessed true and in every way great spirits, uniting gentleness with wisdom in the various chances of life, and in their intercourse with one another. They despised everything but virtue, caring little for their present state of life, and thinking lightly of the possession of gold and other property, which seemed only a burden to them; neither were they intoxicated by luxury; nor did wealth deprive them of their self-control; but they were sober, and saw clearly that all these goods are increased by virtue and friendship with one another, whereas by too great regard and respect for them, they are lost and friendship with them.
By such reflections and by the continuance in them of a divine nature, the qualities which we have described grew and increased among them; but when the divine portion began to fade away, and became diluted too often and too much with the mortal admixture, and the human nature got the upper hand, they then, being unable to bear their fortune, behaved unseemly, and to him who had an eye to see grew visibly debased, for they were losing the fairest of their precious gifts; but to those who had no eye to see the true happiness, they appeared glorious and blessed at the very time when they were full of avarice and unrighteous power.
We have historical records of Socrates, Critias, Dropidas, Solon, and of course of Plato.
Solon was a prominent figure of Greece. He died in 539 B.C. His life was recorded by Plutarch in 75 A.D.
Dropidas was mentioned in the poems of Solon.
Socrates was condemned to death by the government by a vote of 281 to 220.
Plato lived 428-348 B.C. His works have been placed online along with 52 other Greek writers by MIT.
One more thing. We generally say that Plato is our original source. Indeed, it was his writing that awakened the West today to the idea of Atlantis. However, it has now been shown that the remembrance of Atlantis was spread thoughout the cultures of the world.