One Piece

One Piece (glossed in print as ひとつなぎの大秘宝, Hitotsunagi no Daihihō, "The Great Treasure of One Piece", but still pronounced Wanpīsu) 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 previous owner, the Pirate King Gol D. Roger.

The Mystery of the Great Treasure
Nobody outside the Roger Pirates seems to know what it really is or what it looks like.

Given the recurring theme of treasure being something of great personal value (for example Luffy's hat), One Piece may simply be an item that Gol D. Roger valued. In Buggy flashback when he was a member of the Roger Pirates, he discuss with Shanks about the meaning of a treasure, which he claim that the rest of the pirates on the ship have the same idea of a treasure as Shanks.

The closest the Straw Hats came to finding it out so far was when Usopp asked Rayleigh what it was. However at that moment was interrupted by Luffy who voiced a disapproval at finding out now what the treasure was, preferring to find out the nature of One Piece by finding the treasure himself and also stating that he would quit if he were to learn what it is cause he does not want to go on a boring adventure.

The existence of this treasure was seemingly confirmed by Whitebeard, despite speculation. He mentioned that "all of it will be written into history" and "a grand battle that engulfs the entire world" will come when One Piece is found.

Another great mystery which Gol D. Roger take part of is the void century and The Will of D. Those have been a recurring theme surrouding the great treasure.