Atlantis was a kingdom that sank beneath the sea in a disaster centuries ago. It is said that the inhabitants were transformed into Tritons and other sea creatures.
Behind the scenes
The destruction of Atlantis is described as having been 'eons' ago or several centuries ago. The centuries reference would seem to indicate it occurred less than a millennia before QFG5. Eons is less specific, either meaning an indefinite but long period of time (an age), or in geologic/scientific reckoning, often millions of years, a billion years (the latter meaning is 'unlikely' in this situation).