Prior to the development of Ba Sing Se as a large metropolis, it was a small underground settlement carved out of the rocks by earthbenders. As the earthbenders excavated further, they discovered the beautiful glowing crystals and began to mine them; these glowing crystals are now used as a common light source in the Earth Kingdom. Eventually, the city expanded above ground, leaving the Crystal Catacombs empty. Although no one lives there today, the ancient underground city of stone still stands.