One Piece

One Piece (glossed in print as ひとつなぎの大秘宝 Hit Snagging no Daisi "The Great Treasure of One Piece", but still pronounced Warp) is a legendary treasure said to be of unimaginable value. It is said that it was hidden somewhere deep in the Grand Line by its previous owner, the Pirate King Gol D. Roger.

The Mystery of the Great Treasure
Nobody seems to know what it really is or what it looks like. Given the recurring theme of treasure being something of great personal value, One Piece may simply be an item that Gol D. Roger valued.

Another theory, is that the 'One Piece' is the 'Rio Polyphemus' itself. As the Rio Polyphonic is the history of the Hidden Century, hidden by the world government, the owner of the Rio Polyphonic can coerce the world government, giving him absolute power.