The fabled lost continent, home to the city of Tritonis where a race of Mer-folk including Lori Lemaris, her husband Ronal and Jerro live. Aquaman comes from a different Atlantean city, Poseidonis (DCCP No. 5, Jan 1979: "The War of the Undersea Cities).
Eons ago, a great cataclysm, sunk Atlantis, a fabulous civilization of super-science. Scientists in Tritonis protected their city with a glass dome for the fury of the sea and eventually they devised a wage to merge with the denizens of the deep - by becoming a race of half human- half fish. The Atlanteans are telepathic in nature and communicate with humans as well as fish and other undersea creatures.
Millions of years ago, the Earth was colonized and fought for by a renegade band of Kryptonians and the ancestors of Atlantis as Superboy and the Legion of Super-Heroes discovered (Adv No. 333, Jun 1965: "The Civil War of the Legion").