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)

 AnimeRulz is the best site to watch One Piece Special: Adventure in the Ocean's Navel SUB and DUB in HD quality.