Atlantica

Atlantica is a world from the Kingdom Hearts games. It is the home world of Princess Ariel.

Almost completely underwater, Atlantica is a rocky world with immense holes on its surface. Beneath the surface is an entire world-full of fish, merpeople, and other aquatic species that make up King Triton's underwater kingdom. The Heartless that appear in Atlantica are unique to it, such as Sea Neons. In all games, the boss of Atlantica is Ursula (sometimes accompanied by Flotsam and Jetsam).

Atlantica is unique in that Sora takes on a different form while on the world's "surface" (technically underwater). Donald Duck transforms him into a "dolphin merboy," himself into a half-duck half-octopus creature, and Goofy into a sea turtle.

Differences
Atlantica has been the most drastically changing world throughout the Kingdom Hearts games. In each game its gameplay is completely different. In Kingdom Hearts, Ariel joins Sora as a party member and things carry out much like in any other world, with the exception that the controls are different to fit Sora's new form. In Kingdom Hearts: Chain of Memories, Sora doesn't change form at all, and the world is treated just like all other worlds. In Kingdom Hearts II, possibly the biggest change, only two areas are able to be traveled. All other gameplay takes place during song mini-games, including the boss battle. Songs include:
 * Swim This Way
 * Part of Your World
 * Under the Sea

Heartless

 * Kingdom Hearts
 * Sea Neon
 * Sheltering Zone
 * Aquatank
 * Screwdiver
 * Search Ghost
 * White Mushroom
 * Darkball (return trip only)

Characters
Bosses are bolded. First appearances are in italics.
 * Princess Ariel - Kingdom Hearts
 * King Triton - Kingdom Hearts
 * Sebastian - Kingdom Hearts
 * Flounder - Kingdom Hearts
 * Triton's Daughters - Kingdom Hearts II
 * Prince Eric - Kingdom Hearts II
 * Ursula - Kingdom Hearts
 * Flotsam - Kingdom Hearts
 * Jetsam] - Kingdom Hearts