Cayce is good in that he saw reality, the nature of his gift meant however that his information was fragmented, often garbled and mixed.

He's an unreliable source but there's a lot of truth in what he saw, try him if you can stomache his peculiar manner.

Plato learned about Atlantis from Egyptian Priests, they themselves were the caretakers of various historical records that stretched all the way to before the flood.

Quite a few greek philosophers knew of Atlantis but kept silent, ancient egyptian priests gave such knowledge freely as they saw Greek philosophers as enlightened people worthy of imparting the true history to.