Around 18,000 B.C., Atlantis and the continent of Mu, which contained the kingdom of Lemuria, suffered a devastating catastrophe that sank both continents. Atlantis was suffering from civil war and a number of invasions from the nation of Lemuria. Foreseeing that the Deviants would eventually reach and attack the capital city of Atlantis and the city of Netheria, the cities' leaders had huge glass-like domes erected over their cities and fortified their foundations.
To stop the onslaught of Lemurian warriors, King Kamuu opened the magma-pits which were the city's main means of heating. The Lemurian invaders were destroyed, but the magma release triggered a seismological cataclysm which caused a series of earthquakes throughout Atlantis. The Second Host of Celestials, finding the direction of Deviant technology counter productive, retaliated to the Deviants' attack by causing massive nuclear explosions in Lemuria.
The combination of explosions and eruptions sank Lemuria, Atlantis and the whole continent of Mu into the ocean. The Great Cataclysm decimated most of the populations of both kingdoms.
A small handful of Atlanteans loaded a number of animals aboard a large ark and escaped the destruction of Atlantis. The Polarian Eternal who would one day be known as Ikaris led the Atlanteans and animals to safety. Because he did not come into direct contact with the humans aboard the ark, they mistook him for a bird.