Atlantis is rumored to have been an island or continent whose inhabitants had access to powerful magic or technology and which has sunk into the seas. Its existence has never been confirmed by science. In the Sixth World, the Atlantean Foundation is the most important organization trying to find Atlantis.
In Earthdawn/Shadowrun universe, Atlantis usually refers to the island of Thera, capital of the Theran Empire, located in what today is the Eastern Mediterranean Sea. It sank into to sea on August 12, 3113 BC, around the time the Fourth World ended.