Introduced for the very first time in One Piece's Alabasta arc, the Pluton was revealed to be an Ancient Weapon with powers of mass destruction. It played a massive role during the pre-time skip of One Piece in both the Alabasta and the Water 7 sagas. Mentioned first by none other than the former Shichibukai, Crocodile, and the Alabasta arc, the Pluton was revealed to be a massively powerful weapon capable of wiping out entire islands with a single shot. With its powers, Crocodile planned to take over the entire world and become a major force in the New World. Unfortunately for him, Robin did not reveal the location of the weapon and he ended up being defeated by Luffy.
The Water 7 arc, once again, revolved around the Pluton as the main plot point. The ship itself was built on the island at some point during the Void Century. Since then, the blueprints for the Pluton had been passed down from generation to generation and, eventually, they ended up with Franky. Robin and Franky were both central to the storyline of the Pluton before the timeskip and their connection to the weapon was, obviously not going to stop there. With the location of the Pluton being unknown, it was inevitable for it to come up at some point in the story. In One Piece's previous chapter, Nico Robin finally revealed that the weapon was in Wano Country itself.
What Is the Pluton?
The Ancient Weapons are described to be tools of mass destruction that existed during the Void Century. Each one of them is known to be capable of destroying the entire world if let loose. Their names are derived from those of Greek gods and they include Pluton, Poseidon, and Uranus. Although most information about Uranus remains to be largely unknown, fans do know quite a bit about the other two weapons. Poseidon, for one, was revealed in full detail during the Fishman Island arc. There, it was revealed that the Mermaid Princess, Shirahoshi, was Poseidon and that she possessed the ability to control all Sea Kings who could level every single island in the world with ease, destroying it in the process. As for the Pluton, Oda went into some detail about the weapon before the timeskip. The ship has been labeled to be the "World's Worst Ship," capable of mass destruction. It is implied that it was used at some point during the Void Century in the Great War that was fought between the 20 Kings and the Ancient Kingdom.
The powers of the Pluton are terrible enough for the World Government to be incredibly wary of it. In fact, the shipbuilders of Water 7 kept the blueprints in case the actual Pluton that was built 800 years ago fell into the wrong hands, as that would spell the end of the world. Ever since the Great War, the Pluton has been sleeping in Wano country. With the weapon being as powerful as it is, it makes sense why the borders of the country were closed for so long. The Kozuki Clan, who likely had something to do with the Pluton, most likely didn't want the weapon to fall into the hands of the World Government. As such, the country isolated itself from the world and has managed to guard the Pluton for over 800 years. With Nico Robin revealing that the weapon is sleeping in Wano and Sukiyaki concurring with her, it appears as if the big reveal of the Ancient Weapon isn't too far off.
What Could The Pluton Be?
In the Alabasta Arc, it was implied that the Pluton was, in fact, a ship, and the one controlling it would have the greatest power in the world. With it, Crocodile planned to rule over the entire world and hoped that he would become invincible. During the Water 7 arc, it was once again confirmed by Oda that the Pluton is a warship. According to Franky and Iceberg, the blueprints of the weapon were so complicated that it almost felt impossible for anyone to craft a ship like it. Given its power, the Pluton must be a massive ship that could wipe out any country with rather ease. While Oda has painted the idea of it being a warship, there could be more to it than meets the eye. After all, Eiichiro Oda is known for throwing curveballs all the time.
It is also possible for the Pluton to not actually be a ship but rather something that resembles one, and, thus, is referred to as one. A very popular idea is that the Pluton could be Zunesha itself. After all, Zunesha does traverse the waters of the entire world and carries a whole country on its back. In terms of power, Zunesha is known to be strong enough to destroy countries, as was confirmed when its power was needed to open the borders of Wano. With its power, the destruction of all the countries in the world seems rather easy as a major swing of the trunk would be enough to do it. It is also possible that Zunesha has other powers that the fans have not seen yet which could end up being even more dangerous. In that case, Momonosuke, the Shogun of Wano, would end up being its controller and one of the biggest threats to the World Government. Regardless, the Pluton possesses terrific power which the World Government fears and it appears to be close to awakening now more than ever.