This place is an alternate dimension created by the Coven of Thule, a group of powerful wizards who claimed to control Atlantis since its founding. Because of their defeat when it came to power, this council enslaved a good number of Atlanteans and brought them to this dimension that was a replica of the Atlantis of the time.
With the passage of time, the magic that kept both dimensions separate began to decline, and the Coven prepared a plan for Thule to occupy different parts of the Earth, destroying the places where they were deposited. Through the intervention of Aquaman and the Justice League, the Coven was defeated and the descendants of the enslaved Atlanteans were welcomed as refugees from Atlantis. As a result of the loss of the magic that sustained this dimension, it is now believed to be missing.