Atlantis is an underwater city filled with mermaid people. People who can breath underwater anyway. That's at least how Marvel and DC depict Atlantis. Futurama played on that myth but changed it to Atlanta, as in Georgia.
In reality, Plato probably made up Atlantis as an allegory. There's no evidence a continent ever sank, especially one larger than "Asia and Libya combined." Although to be fair, they probably weren't measuring Asia from East Turkey to Eastern Siberia as we do now.
The possibly connection with the city state of Helike adds an interesting twist. Plato knew about the sinking of Helike of course. But for years it was itself considered a myth, and it's only been found quite recently. The truth is about as wild as the story, overall.