One Piece: Umi no Heso no Daibouken-hen
English name: One Piece TV Special: Adventure in the Ocean's Navel
Genres: Action, Adventure, Fantasy
Type: Special, Episodes: 1
Aired: Dec 20, 2000
Studio: Toei Animation
Source: Manga
Rating: PG-13 - Teens 13 or older
Synopsis:
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)