When you're trying to figure out where places are located in the DC Universe, you have to use every scrap of information you can find, including some unusual places.
Superman is dead, and Lois is trying to make herself forget by attending a swanky UN function, where she meets...
What's going on?!?
So now, we not only know where Atlantis is...but as a bonus, we have an alternate dimension with no Aquaman!!
From Lois Lane #43 (1963), as reprinted in Lois Lane #113 (1971)