Atlantis , in Greek legend, large island in the western sea (the Atlantic Ocean). Plato, in his dialogues the Timaeus and the Critias, tells of the high civilization that flourished there before the island was destroyed by an earthquake. The legend persists, and societies for the discovery of Atlantis remain active. Plato described Atlantis as an ideal state, and the name is considered synonymous with Utopia. Francis Bacon called his account of the ideal state The New Atlantis.