Pluton is one of the three Ancient Weapons, alongside Poseidon and Uranus. Pluton is a highly advanced warship capable of mass destruction, labelled the World’s Worst Battleship. The battleship is capable of destroying whole islands in a single shot. It was constructed long ago in the city of Water 7 in the Grand Line.
Kaido and Big Mom both knew something related to the One Piece was in Wano but seemingly never found it. We just found out that Pluton is in Wano, and it has been for a very long time. After being in Wano for decades, Kaido never found Pluton?
Crocodile knew about Pluton, so I bet Kaido does, too. He is in Wano for a reason, he has a strong interest in weapons, and he said he wanted to start the greatest war ever.
We know Pluton is a battleship Franky can build and had actual backup blueprints. Meaning it was built already, and it should be sitting around somewhere. Also, it is “capable of destroying whole islands in a single shot” (chapter 193). How can one battleship instantly destroy an island? Where is it hidden?
Maybe both Kaido and Big Mom were sent deep into the ground for a reason. I think Oda wants to tell us that there is something deep underground. And it’s Pluton. The reason no one has found Pluton is because it’s deep underground. Surely this has been pointed out before and you can take a look at the Wikipedia page for Pluton. In short, Pluton is a general term for bodies of magma.
Down in that same magma chamber could be where the Poneglyphs and seastone were made. It could all be near a huge underground forge. Perhaps the weapon Pluton is a battleship capable of making use of the destructive power of magma. And it’s possible that Pluton is made of the stone that Poneglyphs are made of since it’s basically indestructible.