Atlantis, the fabled undersea continent that was the stuff of ancient legend, and how it existed on two parallel planets. On Earth-One, when the entire continent was ravaged by earthquakes thousands of years ago, it was submerged within the northern Atlantic. So was its counterpart on Earth-Two, however, there were differences.
Whereas the surviving two cities of Poseidonis and Tritonis were protected within domes on Earth-One, the eventuality of their twins cites Aurania and Venturia on Earth-Two were quite different as they were encased within undersea caverns with air pockets. Hence, those inhabitants need not breathe water while their counterparts of Earth-One had that ability thanks to an ancient serum. And it was that serum that mutated the Tritonis race into become mermen and mermaids, such as Ronal (originally a merman from another world) and Lori Lemaris. Whether Manno the Mer-Man was from Tritonis or another undersea enclave has never been revealed, though his own merman race parallels that of a warrior merman race on Earth-Two.
The predominant Atlantean kingdoms of Poseidonis and Aurania had the red-haired queens Mera and Eeras to reign over them, with the Earth-One Aquaman claiming kingship of Poseidonis while his Earth-Two twin refrained from politics and stayed within the open sea. And it was later Earth-Two's cities and the entire continent that had been raised, only to be submerged once more thanks to nefarious Queen Clea of Venturia's malevolent machinations. Hence by the end of the 20th century, all four cities on both worlds remained separated from the surface as they had for centuries before.