Atlantica
Just before you enter this world, a brief scene will show Sora being concerned about entering this world, since they will drown if they enter it. Donald then tells him that his magic can allow them to survive.
Upon landing, you'll see that Sora now has a blue dolphin tail, Donald now has octopus legs and Goofy has become a sea turtle.
You'll then be introduced to Ariel, Flounder and Sebastian, who will teach you how to swim. You mostly have to follow Flounder around. Soon, a bunch of Heartless will appear, and your new friends will hide in a clam. After defeating the Heartless, attack the clam to let your friends out, revealing a save point (type 2). Ariel will then join the party.
Follow the trident marks on the wall to advance with the story. You'll also notice that the Heartless are very different than the ones encountered in other worlds, though the Search Ghosts can also be found in Halloween Town. There are very few Treasure Chests around this world, but there are calms around, which might contain a piece of treasure; just attack them when there are no Heartless nearby to open them. However, three of the clams are in different colors: red, blue and yellow. These clams can only be opened with magic; use Fire on the Red Clam, Blizzard on the Blue Clam, and Thunder on the Yellow Clam. If you have not yet completed Phil's training in Olympus Coliseum, then you'll have to wait until after defeating Giant Ursula to open it.
Once you reach the palace, three Screwdivers will follow the party, but King Triton will vaporize them. He refuses to tell the party about the Keyhole, and will forbid Ariel from leaving the palace. She swims off. You can find her at her Groto.
In the Groto, the music will change, and you'll notice a trident head-shaped hole on the wall. This place has some Treasure Chests, one of which contains a Torn Page, which is needed to complete 100 Acre Wood and get 100% in the game.
Now you must follow the dolphin that swims around the area back to the Undersea Valley. Once you reach that area, grab onto it (make sure there are no Heartless around) and it will take you to the Sunken Ship. Inside of the ship, you'll find a treasure chest containing a Crystal Trident. However, just before you open it, a shark will smash the ship, creating a hole big enough for the party to go through. He'll appear randomly around Atlantica, so be on your guard and heal whenever your HP gets low, as the shark is quite powerful. Now you must return to the Groto. The easiest way is to use a shortcut found near the ship, and use the rapid water sprout to push a treasure chest out of the way.
During the cutscene, King Triton will show up and destroy the Crystal Trident, causing Ariel to swim off crying. He'll also exile the party out of his world. However, the save point at the Tranquil Grotto will not allow you to leave. Meanwhile, Ursula is trying to trick Ariel into helping her take King Triton's trident. Once you reach the palace, you'll notice that King Triton is wounded, and that Ursula has made off with his trident. Ariel is determined to get it back, and will then rejoin the party.
Head back to the Sunken Ship, and you must now fight the shark.
| Glut | |||||
|---|---|---|---|---|---|
| HP | STR | DEF | MP R | EXP | |
| 200 | 22 | 19 | 32 | 100 | |
| Resistances | Weapon | Fire | Blizzard | Thunder | |
| x1.0 | x0.49 | x0.5 | x1.0 | ||
| Gravity | Stop | KO | Other | ||
| x0.04 | x0.3 | x0.5 | x0.5 | ||
| Rewards | Hi-Potion (100%), Ether (80%), Mega Potion (30%) | ||||
Once he is defeated, Sebastian will open a hidden tunnel leading to the Den of Tides. There is a save point at the Cavern Nook, so you'll be able to save and heal. At the end of the tunnel, you'll encounter Ursula.
| Ursula | |||||
|---|---|---|---|---|---|
 |
HP | STR | DEF | MP R | EXP |
| 450 | 22 | 19 | 1 | 1,000 | |
| Resistances | Weapon | Fire | Blizzard | Thunder | |
| x1.0 | x0.0 | x0.0 | x0.0 | ||
| Gravity | Stop | KO | Other | ||
| x0.0 | x0.0 | x0.5 | x0.5 | ||
| Rewards | Mermaid Kick | ||||
After defeating her, she'll disapear into what appears to be a portal. You'll then learn Mermaid Kick, which is needed to reach Ursula for the second battle with her
| Giant Ursula | |||||
|---|---|---|---|---|---|
| HP | STR | DEF | MP R | EXP | |
| 900 | 22 | 19 | 36 | 1,500 | |
| Resistances | Weapon | Fire | Blizzard | Thunder | |
| x1.0 | x0.49 | x0.5 | x1.0 | ||
| Gravity | Stop | KO | Other | ||
| x0.04 | x0.3 | x0.5 | x0.5 | ||
| Rewards | Thunder Upgrade, Ansem's Report 3 | ||||
After defeating Ursula, you'll be taken back to the palace, where King Triton will tell you that the world's Keyhole is in Ariel's Groto. Head to the Groto (don't worry about the Heartless now, since they won't attack the party now), and then Ariel will use the trident to reveal the Keyhole, ending Atlantica's story.
If you want to get a 100% completion, you will need to return to Atlantica to perform Trinity Detect, which can be found in Triton's Palace.