WARNING: The following contains spoilers for One Piece Chapter 1045, "Next Level," by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English from Viz Media.
One Piece is a series of many, many mysteries, and the sacred treasure of Marijoa is one of the most pressing. Ever since it was first mentioned in Chapter 761 by Donquixote Doflamingo, the question of the treasure's true nature has loomed large over the story. At that point, all that was revealed was that it had the power to ''shake the world to its foundations',' and that Doflamingo was able to use this knowledge as leverage over the Celestial Dragons.
It wouldn't be until One Piece Chapter 906 that the treasure was finally shown, but its appearance only raised further questions. Kept in a frozen vault beneath Pangaea Castle, seemingly only accessible by the World Government's true leader Imu, is a giant Straw Hat -- identical to that of Luffy's, other than its immense size.
How exactly a giant Straw Hat could wield immense power in the One Piece world has led to countless speculation, including who the original wearer of the hat might be. The most popular theory is that it belonged to Joy Boy -- a figure from the Void Century who wielded immense influence by having ties to the Ryugu Kingdom, being comrades with the ancient elephant Zunesha, and even potentially being the original owner of the One Piece itself.
That said, the Joy Boy theory would also mean that he has to be a giant to match his headwear. While not out of the realm of possibility by any means, it's an extra wrinkle that remained unconfirmed by the story, meaning the idea was not ironclad.
Recent revelations, however, have added credence to the theory. After nearly dying in Chapter 1043, Luffy regains consciousness and his Devil Fruit awakens -- an event that Zunesha heralds as the return of Joy Boy.
In the next Chapter, the Five Elders discuss the true nature of Luffy's fruit -- that it is in actuality the Human-Human Fruit Model: Nika, an incredibly mighty Zoan Fruit that grants its user the power of the Sun God. Given Zunesha's comment, it seems incredibly likely that Joy Boy was the previous wielder of the fruit.
If this is the case, then the giant Straw Hat finally has an explanation. In One Piece Chapter 1045, Luffy proceeds to test out his newly awakened fruit's powers -- including gigantification, which Luffy humorously uses to play jump-rope with Kaido's body.
If Joy Boy was indeed the previous user of the Human-Human Fruit Model: Nika, then it goes a long way toward explaining why the Straw Hat was so massive in size. After all, Luffy's clothes grew in conjunction with his body -- it's entirely possible that Joy Boy's hat was a regular-sized one that grew thanks to the fruit's powers. Alternatively, it could be that Joy Boy used the fruit to remain in giant form all the time, and the hat being giant was its default state.
While these revelations go a long way to explain why the Straw Hat in Marijoa's vault is that size, much about the treasure remains a mystery. Why it wields such immense political power -- aside from being the headwear of such a notorious figure -- is unknown at this point, as well as why it needs to be kept in cold storage specifically.
Nonetheless, the pieces of the puzzle of Marijoa's sacred treasure are slowly but surely coming together. One Piece may still be a story built on a foundation of many, many mysteries, but with Luffy's awakening, the series has gone a long way to answering readers' many, many questions. As the ''Wano'' arc hurtles toward its climax, no doubt many more revelations are on their way.