One Piece Special: Adventure in the Ocean's Navel
One Piece: Umi no Heso no Daibouken-hen
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)
Seasons
One Piece Special: Adventure in the Ocean's Navel
One Piece Special: Open Upon the Great Sea! A Father's Huge, HUGE Dream!
One Piece Special: Protect! The Last Great Performance
One Piece Special: The Detective Memoirs of Chief Straw Hat Luffy
One Piece: Episode of Nami - Tears of a Navigator and the Bonds of Friends
One Piece: Episode of Luffy - Hand Island Adventure
One Piece: Episode of Merry - The Tale of One More Friend
One Piece 3D2Y: 15th Anniversary
One Piece: Episode of Sabo - The Three Brothers' Bond
One Piece: Adventure of Nebulandia
One Piece: Heart of Gold
One Piece: Episode of East Blue
One Piece: Episode of Skypiea
One Piece: Barto's Secret Room