Ba Sing Se
The city is by far the largest city in the world, essentially being more of a small country than a mere city. Ba Sing Se means "Impenetrable City"; the city is so named for its 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.