Atlantis
Atlantis has appeared in many works of fiction.  In Marvel Comics, Atlantis is at the bottom of the ocean and exists in modern times, with people who breathe water.  Other works of fiction use Atlantis as background.  For example, Robert E. Howard set his Conan the Barbarian stories in a fictional time called the Hyborian Age, which began with the destruction of Atlantis and ended when real written history started.