Growing up, when I thought of Atlantis I thought of it as an existing, living hidden civilization, either under an air dome or populated by mer-people. This is the pop culture take, whereas a great deal of literature and pseudo-history take it as an extinct lost city, populated only in ancient times.
Before it sank into the sea, Atlantis is described as an advanced civilization - sometimes meaning advanced for it's time, sometimes meaning more advanced than modern technology.
In the Golden Age, Atlantis is first mentioned in 1949 in Sub-Mariner Comics 31. This is reportedly the only mention of Atlantis in the Silver Age, and I'm not reading them all to confirm.
Atlantis is mentioned as the ancestors of Namor's people. Atlantis was a surface city with technology more advanced than modern day. Scientists invent a magnet that can attract anything, but the ruler thinks it's a bad idea and throws it into a lake. The magnet goes deeper into the earth, and for centuries Atlanteans dig down after it, until the dig so deep it upsets the foundations of Atlantis, which sinks into the sea.
Some Atlanteans developed the ability to breath water after several generations, though I would think they'd need to be quicker than that.
These people are the ancestors of the current mer-people Atlanteans, and the magnet became the source of Earth's gravity, which is not only wrong but stupid. The city/continent of Atlantis itself is described as lost.
Namor says this is just a story, and a wrong one at that, but there are similarities to later Atlantis depictions.
In the Silver Age, there is a quick mention of Atlantis by Namor referring to a sunken artifact, here Atlantis being a lost city. It comes up again full force in Fantastic Four Annual 1, 1963, where Namor is reunited with his people after unknown years of living on skid row.
There's some backstory given to Atlantis and the evolution of merpeople - here merpeople evolved along a parallel track to humanity, and aren't evolved from the original Atlanteans. In fact, Atlantis is depicted as originating under the sea. I believe this is filled out later, with merpeople settling in the ruins of post-cataclysm Atlantis.
The bit about Namor trying to locate Atlantis confused me (wouldn't he know where it was?), until there was a reference to New Atlantis. So, presumably, the Atlanteans call wherever they live "Atlantis". Atlantis is located in Antarctica in 1920 (which makes Namor in his early 40s here). As of FF Ann 1 it's in the Atlantic Ocean, but the Atlanteans pack up and abandon Namor at the end, so we'll have to wait and see where the settle next.
The first description of pre-cataclysm Atlantis is in an Iron Man comic, Tales of Suspense 43. Atlantis is an advanced, terrestrial civilization threatened by tidal waves and earthquakes. Atlantis is covered by a dome, which protects it as it sinks under the waves, through the earth, and eventually settling in the Earth's core like the magnet in Namor's stupid folktale.
It seems clear that Kala is talking about all of Atlantis, which is city size. This is later retconned/corrected to being a city within the continent of Atlantis, with other parts presumably settling under water.
Atlantis is later involved in a great deal of Marvel pre-history, which, like the hollow earth stuff, will get a lot more convoluted as we go on.
