This is part 2 of a 4-issue arc that introduces four new cities: Earth, Water, Air, and Fire. The first issue of this arc, #575, introduced the city of Earth.
In #576, the team puts on their parkas and ventures to Antarctica, where Advanced Idea Mechanics is trying to uncover a hidden city. The FF have to compete with the bee-keeper scientists to claim discovery, and defend the city against AIM along with the city’s protectors: Humanoid fish.
Yes, it’s Atlantis. The City’s king, Ul-Uhar, is grateful for the FF’s intervention and give them a device to enable future communications. Susan Richards is named as humankind’s ambassador, and tries to reach Namor to let him know that a more ancient version of Atlantis has been discovered. She calls it “Old Atlantis.”