The oldest mention and probably origin of Atlantis is Plato in his dialogues Timaeus and Critias (around 360 BC).

In Critas, which he never really finished, the lead character talks about the geography of Atlantis as well.

Let me begin by observing first of all, that nine thousand was the sum of years which had elapsed since the war which was said to have taken place between those who dwelt outside the Pillars of Heracles and all who dwelt within them; this war I am going to describe. Of the combatants on the one side, the city of Athens was reported to have been the leader and to have fought out the war; the combatants on the other side were commanded by the kings of Atlantis, which, as was saying, was an island greater in extent than Libya and Asia, and when afterwards sunk by an earthquake, became an impassable barrier of mud to voyagers sailing from hence to any part of the ocean. The progress of the history will unfold the various nations of barbarians and families of Hellenes which then existed, as they successively appear on the scene; but I must describe first of all Athenians of that day, and their enemies who fought with them, and then the respective powers and governments of the two kingdoms. Let us give the precedence to Athens.

The Pillars of Heracles were the antique name for the Strait of Gibraltar, so being outside would put them into the Atlantic. And apparently the island was greater in extent than Libya and Asia Minor combined. Asia Minor is today referred to as Anatolia and is essentially Turkey (map of Asia Minor provinces, 500 BC). Libya on the other hand was the Greek's "third continent" after Europe and Asia, the boundaries are not exactly clear but here is a map by the "father of history" Herodotus, from around 500 BC too.

So if we were to believe Plato's account you can guess how vast it would have been, completely dominating the Atlantic ocean, which seems unlikely to fit geographically if you look at a map of the northern Atlantic ocean. It could possibly fit on the Mid Atlantic Ridge but then that's quite a distance from Gibraltar so the Atlanteans must have had incredibly advanced seafaring technology to even reach mainland Europe, and all that in a time 9000 years before the story written in 360 BC, so a good 11500 years from today - a time where your grandfather still had stories about the Ice Age, where the first stone structures in Jericho were just in their planning phase, where Mesopotamians started to discover beer, where the Magdalenians in France went "You know what? I could go for some cave pictures of us hunting animals." for the first time and the Azilians in the Basque region only bagan painting pebbles.

All in all Plato's account seems highly unlikely, doesn't it?