Ancient Weapons

The Ancient Weapons (古代兵器) are three weapons capable of mass destruction, each taking a different form. They are known as Pluton, Poseidon, and Uranus. The weapons are not limited only to inanimate objects, but consist of living beings as well.

The Ancient Weapons are the argument used by the World Government to forbid research of the Void Century, since they fear that knowledge of the weapons could result in a global war. However, this hasn't stopped members of the World Government, most notably Spandam, from attempting to gain one of the weapons for themselves. Even without knowledge of the missing years, their power has been known to entice casual observers (as was the case with Vander Decken IX's interest in Shirahoshi, otherwise known as the Ancient Weapon Poseidon).

Pluton
While Pluton has yet to appear in the story proper, it has been referred to as an ancient ship capable of massive damage and destruction. The blueprints to recreate Pluton were held by the Fishman carpenter, Tom, of Tom's Workers. Before his death, they were handed down to his student Iceburg, who then subsequently gave them to his colleague and Tom's fellow student Cutty Flam, who destroyed them after having memorized them. While the exact whereabouts of Pluton remain unknown, the Alabasta Poneglyph contains information on its location.

Poseidon
Poseidon was first mentioned when Nico Robin read the Poneglyph on Skypiea. However, details of the weapon were not delved into until much later, when she reached Fishman Island. Poseidon was a Mermaid Princess who lived in the Void Century that had the power to talk to Sea Kings. Princess Shirahoshi also has this power, and as stated in a conversation between Neptune and Robin, she has now inherited the title "Poseidon".

Uranus
Nothing is currently known about Uranus, except for its name. Given the nature of Pluton and Poseidon, Uranus could easily be an animate or inanimate object.

Trivia

 * Each of the Ancient Weapons is named after a Greek god who are affiliated with the earth, sea, and sky respectively: Pluto ruled the underworld, which was believed to be underground and all mineral wealth within the earth belonged to him; Poseidon was the ruler of the sea; and Uranus was the personification of the sky.
 * In the One Piece Film Z it's revealed the trump card of Marines "The Dyna Stone", compared to the Ancient Weapons for its power.