The Island of Atlantis is where all Atlanteans once lived in peace and harmony a long time ago. It was located in an unknown area in the middle of the ocean, above the surface, until it sank.
The Atlantean architecture is very much like those of the ancient civilizations of Greece and Rome, with tall pillars carved in marble and buildings with triangular roofs and domed ceilings. This may indicate that Atlanteans either existed around that era or even before the classical civilizations. This would also explain why Fu Dog, in "Hero of the Hourglass", needed to travel back in time in order to see the race's birthplace.