The Kingdom Hearts Atlantica world is originally from the 1989 Disney film 'The Little Mermaid'. In the film the young mermaid Ariel must save her underwater colony from the evil sea witch Ursula. During the film she also meets a prince named Eric, who becomes Ariel's love interest, and who she eventually ends up with in true Disney fashion.

Though the look and feel of Atlantica was originally created largely by Disney, the actual story of The Little Mermaid was based on a fairytale written by 19th century author named Hans Christian Anderson. Atlantica is a rocky world with lots of hidden places and crevices, including Ariel's secret cupboard, where she keeps all of the human things that she finds, and is utterly fascinated by (such as basic kitchen cutlery). The areas remains very similar to the land that we saw in the film, and the game's creators clearly took a lot of time trying to replicate the scene as best as they could. Atlantica is the only level in Kingdom Hearts where you are underwater, and must swim everywhere. This gives the level a new dimension, and gives the game some variety.

In Kingdom Hearts, Atlantica is an underwater world that you must visit as part of your quest in order to get to the Hollow Bastion and rescue your friend Riku. You must visit this world because it has, like most of the places you visit in the game, been taken over by the Heartless, who are the evil and dark enemies of the game. You must first find King Triton (the ruler of Atlantica) who will warn you of Ursula before sending you on your mission.

Kingdom Hearts Atlantica appears in both Kingdom Hearts games, and both times you must defeat Ursula in order to restore peace and vanquish the Heartless.