One Piece

One Piece (ひとつなぎの大秘宝（ワンピース） Wan Pīsu?, kanji read as "Hitotsunagi no Daihihō", translating to "The Great Treasure of One Piece") is a legendary treasure said to be of unimaginable value.

It is said that it was hidden somewhere deep in the Grand Line, presumably in the last island, Raftel, by its deceased owner, Gol D. Roger(past) and now Monkey D. Luffy's ownership.

The Mystery of the Great Treasure
Aside from the Roger Pirates and Straw hat Pirates, before their deaths, no one actually knows where the great treasure is.

The closest the[Straw Hat Pirates have ever come to finding out the nature of One Piece was during the Sabaody Archipelago when Usopp tried to ask Silvers Rayleigh about it. However, Luffy stopped Usopp from discussing the treasure on the grounds that it would defeat the purpose of their adventures, and that becoming the Pirate King would have little merit if he already knew about the One Piece.

After constant speculations and doubts, the existence of the treasure was confirmed by Whitebeard with his last breath. He mentioned that "a grand battle will engulf the entire world" and "the world will be shaken to its core" when One Piece is found.