Over the 500 hundred years since the Rupture, when an Avatar-gone-bad nearly destroyed the world, the land of Avatar has changed immensely.
This post-Rupture world is very different from the pre-Rupture world. For example, the only significant places of old that are completely unaffected are Ba Sing Se, the Avatar Temples, the Air Nomad Temples, and the Spirit Grove in the Northern Water Tribe.
All the islands that surrounded that island the Northern Air Temple is located on crumbled and sunk during the Rupture, leaving a free standing mountainous island and clearing space in the ocean. More than half of the island the temple was on also fell into the watery depths. The Air Nomads have expanded the Northern Air Temple, tripling the size. The same has occurred throughout the rest of the Air Temples.
Ba Sing Se, being the impenetrable city, still managed to survive during the Rupture, even though half of the Lower Ring, most of the Middle Ring, and the Royal Palace were destroyed. They persevered and have made the city larger, grander, and better defended for Rupture like events. Much of the rest of the Earth Kingdom was destroyed, but they managed to recreate their towns and cities. Half of Omashu slid into the abyss it is surrounded by. However, the king of Omashu survived and began to construct a better city that was supported by stone beams going from the city across the abyss to the mainland, reinforcing the island it was on. The peninsula on which Wulong Forest was located shattered, crumbling into several small islands. The rest of the Earth Kingdom is still rebuilding after the devastating Rupture.
Most of the Fire Nation sunk during the apocalypse, but the main island survived, though it shifted south one hundred miles, and a whole different archipelago arose from the ocean, extending further south and southeast instead of east. Several more islands rose to the east, forming a sort of barrier around the main island. The Fire Nation grew larger and the land was more fertile and inhabitable than before.
The northern ocean froze during the Rupture, and afterwards earth rose from the ocean floor. Fires melted the ice of the eastern Northern Water Tribe, and caused half the island to be lost in the waves. But the new land extended the tribe westward, allowing for a new chance to recreate what they had lost. Down south, the whole continent shifted west as another, smaller continent budged up from the ocean. The tribe doubled its size and claimed the island for itself.
So, I hope you have a better picture of what the world looks like, and I hope you enjoy the story.
