This is the fragmentary dialog of Plato which contains the only known ancient account of Atlantis. If you have only read out-of-context excerpts, you should read this short document in its entirety and make up your own mind whether Plato meant the tale literally or as part of an allegory about an ideal society (as Critias implies early on in the dialog, refer to: "All that is said by any of us can only be imitation and representation.") Quite interesting
http://www.sacred-texts.com/atl/critias.txt