Atlantis is a location, based on the mythological lost city of the same name.
Atlantis was a city that fell into the sea because the Atlanteans were too focused on building death traps to construct a proper foundation. As a result, the entire city is upside-down, rendering most of the aforementioned death traps useless, as seen in the pilot episode. Atlantis's sinking also makes it only accessible when there is snow around the Drake Barrier Reef, causing a change in currents.
Such a weather phenomenon occurs for the first time in fifty years when DuckTales kicks off, and both Scrooge McDuck and Flintheart Glomgold go with their respective groups to find the city. Atlantis was also home to the Jewel of Atlantis - a gem that served as the power source for the underwater city and its death traps - until Scrooge and his family took it. Flintheart Glomgold later destroyed Atlantis in a futile attempt on Scrooge's life, leaving his employees stuck at Atlantis.