Crystal Catacombs

Underneath the palace, and possibly much of the city, lies old Ba Sing Se, an ancient underground city deep within the earth. In old times, Ba Sing Se was once simply a small subterranean city carved out of rocks by Earthbenders. As the benders further excavated, they came across beautiful illuminated crystals and began to mine them. The crystals currently serve a common light source for Earth Kingdom inhabitants. Eventually, as the metropolis' expansion reached above ground levels, the catacombs were abandoned.[4] Inside the interior of Old Ba Sing Se still lies ancient stone structures, a waterfall and pool, and large green crystals that populate many of the catacombs.