Pluton

Pluton 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. For that reason, Sir Crocodile sought to gain control over this weapon for his own means. 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.

Later in the Water 7 arc, it was revealed that the blueprints for the weapon have been kept in existence in 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 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, stating the world will just have to find another way to destroy Pluton if it is ever revived. Considering it was constructed at Water 7 its most likely that Pluton is a warship.