Atlantica

Almost completely underwater, Atlantica is a rocky world with immense holes on its surface. Beneath the surface is an entire world full of fish, merfolk, 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).
King Triton is the ruler of this world.
The keyhole to this world is hidden in Ariel's grotto.

Atlantica has been the most drastically changing world throughout the Kingdom Hearts games. In each game it appears in, 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 is seen; only two areas are able to be traveled and the story is not progressed by battling enemies, but by singing.
Kingdom Hearts gives Atlantica a sprawling design that requires certain special abilities to navigate successfully. Sora and company arrive in the Tranquil Grotto, where Ariel and Flounder host the swimming tutorial. From there, the party travels forward, practicing battle in the Undersea Valley, following the trident marks to their final destination. One exit leads to the ironically-named Calm Depths, which plays host to a swift moving current that pushes Sora and his party around unless they use Ariel's dolphin friend or obtain the Mermaid Kick ability. The Mermaid Kick ability replaces the Dolphin originally required to get through the calm depths. Following the trident mark here takes the group to the Undersea Cave with a strong current of its own. Making it past the current leads to the Undersea Gorge, a vast area which leads to several other places in Atlantica. On the opposite side of the gorge are the entrances to Ariel's Grotto and, under the crate, the end of a hidden tunnel that leads to the Sunken Ship. Somewhere in the middle is the entrance to Triton's Palace and the Throne Room within. Also there, but easily missed, is the Undersea Garden, hidden to the left of the Palace entrance.
After getting into the Sunken Ship area, Sebastian helps the party find a hidden door in the corner which leads to the creepy Den of Tides; the right fork leads to the Cavern Nook, a room with a save point, while the left fork takes the party to the Tidal Abyss and, past that, Ursula's Lair for the first of two encounters with the sea witch. Finally, using Mermaid Kick from the Calm Depths, Sora and the party can reach the very end and find the entrance to the Open Ocean, where their final encounter with Ursula takes place.