Ba Sing Se is the titanic capital of the Earth Kingdom, wherein it encompasses nearly all of the northeastern corner. It was the last great Earth Kingdom stronghold after the fall of Omashu.

The city is by far the largest city in any of the Four Nations, essentially being more of a small country than a mere city. Ba Sing Se means "Impenetrable City"; the city is so named for its two world-famous walls, the gates of which have no hinges or other hardware, and are opened by use of Earthbending. Behind the Outer Wall lies a large expanse of farmland, Lake Laogai, and several mountains. The Inner Wall encloses the city itself. The city is also notable for having a unique system of dividing the populace into different parts of the city based on social status.

Ba Sing Se was the safest place for war refugees because of the Fire Nation's failure to penetrate its walls; however, it was later captured by the Fire Nation. In one of the final battles of The War, Ba Sing Se was liberated by the Order of the White Lotus in the Liberation of Ba Sing Se.