Plato wrote of a philosophy in which the material world that we perceive with our senses is less real than an invisible intangible world of thoughts and thought forms.
Plato wrote about this philosophy primarily through dialogues most of which, as far as anyone can tell, did not take place and could not possibly have been heard first-hand by Plato if they did.
He claimed the philosophical ideas he wrote about originated from the muses. You know, the nine Greek goddesses that fly around giving people ideas.
Writing about things that are purely nonexistent (in the mere material world, at least) is what Plato is famous for. Which is just as well, because (like everyone in his time) he knew bugger all about how anything in the material world actually works.
Still, despite all that he might have had access to reliable sources of historical information about Atlantis which he reported in one of his otherwise fictional dialogues. What were those sources, again?