Atlantis is a small island-continent in the ocean west of Thuria, and east of the Pictish Islands. It's the home of the Atlanteans, enemies of the Picts and the civilized Seven Empires of Thuria. They established colonies on Thuria itself. It is hardly a coincidence that "Commoria" in the days of Kull resembles "Cimmeria" in Conan's lifetime. The Atlanteans once declared war against a mountain-city in "The Altar and the Scorpion". It was the homeland of Kull. Priests of Xultha were sent from Na-hor into Atlantis.
During the Cataclysm which ended the Thurian Age, Atlantis sunk beneath the waves. However, many Atlanteans escaped the destruction on ships and joined their continental colonies. Here, these survivors were forced back into the stone age by a lack of resources. Still, they became skilled in this medium and developed an artistic culture. The Atlanteans soon entered a war with the surviving Picts. However, superior numbers and a stone age level of military technology granted the Picts a major victory. The survivors of this war devolved back into ape men. They eventually re-evolved into a race of humans known as the Cimmerians of the Hyborian Age.
Fortunately, there were a few survivors of the Atlantean catastrophe. The largest concentration of survivors was in the southern city of Netheria, whose priests and intellectuals foresaw an attack by the Lemurians. They enclosed their small city inside a dome of an unknown glass-like substance, before excavating and fortifying their city's foundation. Consequently, when the continent of Atlantis fragmented, Netheria remained intact as it sank, coming to rest within a vast undersea cavern.
Tiny bands of Atlanteans managed to find refuge on neighboring islands and the Thurian continent, which were spared from the Cataclysm. One group of refugees fled to Antillia, a group of islands in the Western Ocean. Other Atlanteans inhabiting the mainland eventually became the tribes of Cimmeria, which the man known as Conan the Barbarian would later hail from. The nearby island of Attilan, home of the genetically-advanced but reclusive inhumans, also survived the Cataclysm virtually unscathed.
About 500 years after the founding of undersea Atlantis, a group of Homo mermanus left the city and made their way towards the Pacific Ocean. This group founded the city of Lemuria on the opposite end of the now-submerged continent of Lemuria, once settled by the Deviants. Following the acquisition and worship of the Serpent Crown, a magical artifact from pre-cataclysmic Atlantis, their complexion and features took on a more serpentine cast, thus differentiating the Lemurians from their Atlantean forebearers.