Atlantis is a legendary place, found by Pearl and her friends. There, they meet Cyrus, apparently hospitable and helpful, but, in the course of the time he proved to be a mean man and showed the true purposes of his civilization.
By what is seen in the ancient world map of the Chapter 19th, Scene 2, the Atlanti continent is in the south pacific, nearby Artemis Island.
Atlantis has always been a technollogically advanced nation, and they always had a desire to fly, however, to achieve that, they betrayed Shambala's confidence and declared war on them looking for gold. They created a machine (or even a weapon) that controls the density and would endanger the world. Their plan backfired and Atlantis sank to the depths of the sea.