Crystal Catacombs

The Crystal Catacombs are huge chambers made of rock and crystal located deep beneath the Earth King's palace; the crystal catacombs are a part of old Ba Sing Se.

History
Long before Ba Sing Se was the huge metropolis that it was in 100 ASC, it was a small underground city 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 still stands.

Years later, the Crystal Catacombs were mainly used as a prison by Dai Li agents. Katara ended up here on the orders of Princess Azula after she was overpowered by Ty Lee and her chi blocking abilities; much to her dismay, it wasn't long before Zuko was thrown in this underground prison as well. Katara lashed out at him during their stay here, but when she learned that the Fire Nation was also responsible for the loss of his mother, she warmed up a bit toward him. Meanwhile, Aang and Iroh worked together in order to free the duo; the Avatar used his earthbending to tunnel through the stone, while the firebender provided light and guidance. After breaking through the wall, Aang and Katara took off, while Iroh stayed to talk with his nephew. During the wise uncle's attempt to persuade Zuko to choose good, Azula barged in with her Dai Li henchmen and trapped Iroh in a crystal encasement while she tempted the banished prince with a promise of redemption and restored honor if he chose to help her instead.

The ancient underground city of old Ba Sing Se was moments later the site of the battle that would complete the Coup of Ba Sing Se, and the Fire Nation's dominance over the Earth Kingdom. Avatar Aang, aided by Katara, fought Princess Azula and Prince Zuko, who had chosen to betray his uncle. The ancient city suffered some moderate damage during this conflict, in which the advantage swayed from side to side; it was only when Azula's Dai Li agents arrived that the battle turned for the worse. Upon seeing the large threat facing him and Katara, Aang decided to unlock his seventh chakra and thus the Avatar State, but before he could take down his enemies, he lost the battle and his life when he was shot in the back by Azula's lightning. Katara used the old city's waterfall to bring Aang's body to safety while Iroh held their attackers back.

Prison cell
The part of the ancient city that was used as a prison appeared to be nothing more than a large cave where old stalagmites and stalactites have joined to form large, strong pillars designed to support the weight of the surface. The walls were embedded with many glowing crystals, providing light for the prisoners.

Old Ba Sing Se


The old city is composed primarily out of rock houses; some of them were just excavated out of the wall, while others were built against the wall. All of the houses that remained in the old city appeared to be very crude in design: One large hole served as an entrance and one or more of the smaller holes functioned as windows. There weren't any signs of other decorations, but it is likely that there were in the city's high days. Many stairs led up from the paved floor to the numerous houses that were built in the wall.

In the center of the large cave that served as the city's main location, was a river. The old earthbenders had used their bending to shape the course of the river into a square, as to provide water to all corners of the large cave. The water itself made its way into the underground cave through a waterfall at one of the sides.

At regular intervals, there would be a large earth column to support the heavy roof and prevent it from caving in. The light of the city was provided by the natural crystals that were spread all over the walls and floor.

Trivia

 * The glowing crystals may be a variety of jennamite, which also has an ability to grow.