Kingdom Hearts 1 - PS2 - Atlantica Part II

Undersea Gorge
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Quick cutscene, then go to the Ariel's Grotto (where Ariel points). Don't
miss the Torn Page. From there, exit out, and watch another cutscene...
Finally, make your way back to the Undersea Valley, clearing out all the
Heartless along the way.
Undersea Valley
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First, clear out all the nearby Heartless (and there are a lot of them), then
grab the dolphin swimming around, and he'll bring you to...
Sunken Ship
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Go around to the side of the ship to enter. Mind the shark. Get the Trident
from the clearly visible chest, then ransack the ship before leaving. As you
leave Mr. Shark returns, but he should be a piece of cake for you. Make sure
you grab all the chests in the area, and then you should be able to guess
where to go from here: Ariel's Grotto. Follow the Tridents if you get lost.
Ariel's Grotto
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Once in Ariel's room, approach the symbol on the wall, cutscene... Then,
you guessed it, Follow The Tridents, then go to Triton's Throne. After the
next cutscene, head all the way back to the Undersea Valley again, save
if you will, grab the Dolphin again, and go back to the Sunken Ship.