Atlantis is the subject of a legend about an advanced island civilization that was destroyed or lost. Stories about Atlantis are first mentioned in Plato's dialogues Timaeus and Critias, in which characters say it was destroyed by an earthquake or a tsunami about 9,000 years before the time in which Plato wrote.
Some people believe that the stories are fictions made up to serve the purposes of Plato's dialogs. But others take them as if they were serious historical accounts.
Some cultures have "lost civilization" myths. In some cases, it has been argued that there is a common historical event or real "lost civilization" at the root of some or all of these legends, but there is considerable disagreement between the competing hypotheses. It may be that these legends have arisen from many different historical events, and are only just now being associated by modern "theorizers" because of their similarity. It may also be that these legends are entirely fictional, but for some reason have arisen and remained popular in many different cultures at different times.