One Piece: Umi no Heso no Daibouken-hen

The Straw Hats discover a bizarre settlement hidden inside an enormous ocean whirlpool, a place known as the Ocean's Navel. Its people live under constant attack from gigantic monsters said to guard a treasure capable of granting wishes, and the promise of such a prize quickly draws Luffy and his crew into the mystery.

Defeating the creatures does not solve the island's problem. Instead, the crew's interference releases a more dangerous force that had been sealed away, turning a treasure hunt into a race to protect the inhabitants from the very evil the guardians were keeping contained.