First mentioned in the 4th century BC, Atlantis is described by the philosopher Plato as an island-state with an ideal society. Becoming corrupt, the Atlanteans waged war against Athens, only to vanish in a cataclysm instigated by the gods, and Plato's compelling tale of an empire sinking beneath the waves has sparked endless debate over whether Atlantis really existed. Presenting the history of this concept, Stephen Kershaw argues that Plato meant it as an allegory—and a warning—for civilizations that become preoccupied with luxury and power.