Whaletail Island

Whale Tail Island is a large island situated off of the southern coast of the Earth Kingdom. The island gets its name from its distinctive shape, which on maps bares an uncanny resemblance to a whale's tail. While little is known about the island or its inhabitants, Water Tribe sailing lore makes the claim that whales can never be found anywhere near the island due to thinking that it's the actual tail of the largest whale in the world. The white coloring of Whale Tail Island on the world map suggests it is or once was Air Nomad territory.

When the Dai Li attempted to get the Avatar and his friends out of Ba Sing Se, they did so by luring them to a stable where Appa had supposedly been kept, and had one of their agents, disguised as a janitor "Old Sweepy" remark in an offhand way that the Bison had been taken to this Island. They discovered the true nature of this plot, however, through contact with Jet, and they deduced that Appa was actually in Lake Laogai.

Fire Nation Settlement
In The Southern Raiders, it was revealed that this island is also where the Southern Raiders have been stationed for some time, and where Katara and Zuko discover that the commander of the Southern Raiders who raided the Southern Water Tribe, Yon Rha, retired 4 years earlier.