Prison rig

The Prison Rig was an offshore prison for Earthbenders, who would be forced to build and repair Fire Navy ships. Most of the Earthbenders on the rig were captured from Fire Nation occupied villages in the Earth Kingdom, where any resistance immediately earned them indefinite imprisonment on the rig. It was run by the Warden. An enormous wall bisects the structure. On one side was a shipyard where the Fire Nation repaired and refueled its ships. The other side was a prison for the captive Earthbenders. The prisoners were not able to bend because the metal rig is surrounded by water, and they are miles away from land. In addition to this, no Earthbender (with one exception) had ever learned to bend metal, further hindering their bending prowess.

History
The Fire Nation Prison Rig was presumably built during the War, off the shores of the Western Earth Kingdom, which has been thoroughly conquered by the Fire Nation. It has built many ships for the expansive Fire Navy, and imprisons hundreds of Earthbenders. Any Earth Kingdom citizens who are caught Earthbending, against Fire Nation law, are considered a threat to the occupation and subsequently sent to the prison rig to silence their bending.

Many prominent Earthbenders from the Mining Village were sent to this rig, including Tyro, Haru's father.

When Haru, an Earthbender, was imprisoned and taken to the prison rig, Katara tricked the local authorities into arresting her by pretending to Earthbend in an attempt to locate and save Haru. In the prison, she tried to encourage the Earthbenders to fight back and regain their freedom, but they had lost hope and decided not to fight the guards.

Team Avatar came to rescue Katara, but she refused to leave until the other prisoners left too. Sokka devised a plan where Aang would block up all the air vents except one, went into the boiler room and pushed the coal up, releasing it to the only available air vent where the Earthbenders were situated. Katara then urged the Earthbender prisoners to fight. At first, the Earthbenders declined again, their spirits still broken. Suddenly, Haru threw some coal at the Warden, who soon tried to attack Haru with fire, but was blocked by Tyro and three other Earthbenders. The rest of the prisoners soon attacked the guards, pushing them back. Finally, the Earthbenders threw the Firebenders into the ocean. After the victory, the Earthbenders used the stolen ships to return to their homeland, swearing to retake their nation from the Fire Nation invaders.

During the battle on the rig, Katara lost her necklace and could not return to retrieve it. It was later found by Prince Zuko, who used the necklace to track down Katara, and subsequently, Aang.

Since the Fire Nation ultimately lost the war with the defeat of Fire Lord Ozai, it is unknown what will become of the massive prison rig. It may have been dismantled, surrendered to the Earth Kingdom, or refitted for peacetime duties.

Trivia

 * It is possible the Earthbender rebellions mentioned later on in Zuko's flashback of Ozai's war meeting were precipitated by the escape of the reinvigorated Earthbenders from this rig, as Tyro said "we're going to take back all our villages".