One Piece: Umi no Heso no Daibouken-hen

ワンピース ルフィ落下!秘境・海のヘソの大冒険

The Straw Hats encounter a city in the middle of a whirlpool, called the Ocean's Navel, that is being destroyed by giant monsters. These monsters supposedly protect a treasure capable of granting wishes, but in the process of defeating these guardians, the crew ends up releasing another evil. (aired after Episode 53) (Source: Wikipedia)