Pluton

Pluton (プルトン) 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. Not seen yet in the series, Pluton was first mentioned in chapter 202 and episode 123.

Alabasta
Crocodile sought to gain control over this weapon in order to be able to defeat the World Government after taking over Alabasta. The Pluton was known to be Crocodile's greatest goal for his start of turning Alabasta into a military state.

The location to which Pluton was taken is said to be somewhere in Alabasta, and the Poneglyph which can be used to find it is also located in Alabasta, 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. They have been passed down in a line of incredible shipwrights for centuries. They eventually fell 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. Also, they are supposed to use 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 Cutty Flam, Tom's other apprentice, had them in his possession all along. They captured both Nico Robin and Cutty Flam, now renamed Franky to stay hidden, in an attempt to acquire both the keys to reawakening the weapon.

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. Franky destroyed the plans to make sure that Spandam would never revive the weapon. With the blueprints destroyed, nobody would ever be able to create a second Pluton.

Trivia

 * Pluto (Πλούτων) is the god of the underworld and Roman counterpart to the Greek god Hades (Άδης).