Atlantis in Golarion?

[Iammars]

I've been coming up for some homebrew ideas in Golarion, and as part of my brainstorming I came up with, I would like the PCs to visit an underwater city in the vein of Atlantis. Does this exist in Golarion already, or should I start planning on making this up?

[Noir le Lotus]

Check for Azlant

[David knott 242]

And in case you don't come across them in searching out Azlant, the Gillmen would be the aquatic humanoids in that underground city, with rather obvious secret rulers manipulating them.

[Therrux]

Check out the adventure path #56. It has information on the oceans of Golarion as well as the intelligent beings that live in them.