Ozai's airship base

Ozai's airship base was located on a remote volcanic island off the coast of the Earth Kingdom. The Fire Nation used the island to house the construction and preparation of a fleet of airships in anticipation of the day of the arrival of Sozin's Comet. The location of the island suggests it was chosen for its proximity to the Earth Kingdom mainland as a strategical advantage in Fire Lord Ozai's planned destruction of the Earth Kingdom. The base itself was capable of accommodating over a dozen airships as well as all the associated crew and military, amounting to thousands of inhabitants at the time prior to the fleet's departure.

History
Prior to the Battle at Wulong Forest, Toph, Suki and Sokka arrived at the shore of the island but found that they were too late to intercept the fleet. As the airship armada began to launch, Toph launched the trio on to the nearest airship, which they were able to commandeer and use to stage a sabotage of the entire fleet.

It is unknown what became of the island after Ozai had been overthrown and succeeded by his son, Zuko. Fire Lord Zuko's intentions for international harmony and reconciliation would almost certainly see that the base was no longer serve its initial function, though it potentially could have been used to house civilian airships.