Pluton

Pluton (プルトン, Puruton) is an ancient weapon (said to be a huge warship) of mass destruction, constructed long ago in the city of Water 7 in the Grand Line. It is said to be capable of destroying whole islands. Pluton is aptly named after Pluto, the roman god of death.

Arabasta
Sir Crocodile sought to gain control over this weapon in order to be able to defeat the World Government after taking over Arabasta. The location to which Pluton was taken is said to be somewhere in Arabasta, and the Poneglyph which can be used to find it is also located in Arabasta, within its royal tombs. The Poneglyph was discovered by Nico Robin but she lied to Crocodile about what it said. Crocodile, however, sees through Robin's deception and stabs her, having judged from Cobra's stunned reaction to his mentioning "Pluton" that the kingdom held the key to finding the weapon.

Water 7
When the Straw Hat Pirates arrived in Water 7, it was revealed that the blueprints for the weapon had been kept in existence at Water 7 all this time, passed down in a line of incredible shipwrights for centuries, and eventually into the hands of the legendary shipwright Tom. The duty of the shipwrights was to guard the blueprints and prevent them from falling into the wrong hands, as well as using them to construct a second Pluton should the first weapon ever get revived.

CP9 sought to take them from Iceburg, Tom's apprentice who had inherited the blueprints. However they later discovered that Franky, Tom's other apprentice, had them in his possession all along. Much later, in Enies Lobby, Franky chooses to destroy the blueprints for the weapon once and for all after seeing the Straw Hats declare war on the World Government. Franky realized that Robin did not want to revive the weapon, and betting that the Straw Hats would save her and that once she was freed Spandam would lose the only means he had of reviving the weapon.