Under da sea~.

North Atlantis

It was originally a small continent with human settlements but this all changed 21,000 years ago during an event known as the Great Cataclysm and it was swallowed by the depths of the sea. This event was caused by King Kamuu when he tried to defend it from the Deviants of Lemuria by opening the city's magma pits. Several settlements survived including Atlantis and Netheria, as they had built a glass dome to shield each before this catastrophe took place.
Sole survivors included such beings as Varnae, Stygyro and Dakhimh the Enchanter. But it was only 13,000 years later than the city of Atlantis was re inhabited, as it was discovered by some Homo Merman's who made it their first mighty city, and becoming the Atlanteans.
After 500 years living in Atlantis another city was rebuilt, Lemuria, by Atlanteans who left Atlantis. And so it was that the Lemurians differed from their former people when they discovered the Serpent Crown in their ruins and they began to develop a more serpentine appearance.
Homo Sapiens and Homo Mermanus have interacted several times, usually with hostility as the Homo Mermanus have invaded the surface world multiple times. Though the current ruler of Atlantis, Prince Namor fought against the Axis Powers in World War II.