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 and export them, gaining immense riches from the commodities as their trade expanded across the world; 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.
In 100 AG, the Crystal Catacombs were used primarily 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, 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 became less hostile toward the young prince. Meanwhile, Aang and Iroh worked together in order to free the two; the Avatar used his earthbending to tunnel through the stone as the firebender provided light and guidance. After breaking through the wall, Aang and Katara took off while Iroh stayed to speak with his nephew. During the wise uncle's attempt to persuade Zuko to choose good, Azula barged in with her Dai Li henchmen, who trapped Iroh in a crystal cage. 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. During the battle, the upper hand reversed several times; it was only when Azula's Dai Li agents arrived that the battle turned in her favor. Upon seeing the serious threat facing him and Katara, Aang decided to unlock his seventh chakra and enter the Avatar State. Before he could take down his enemies, however, he was shot in the back by Azula's lightning, consequently losing the battle and his life. Katara used the old city's waterfall to bring Aang's body to safety while Iroh held their attackers off long enough for them to escape, before being arrested himself.
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 had joined to form large, strong pillars designed to support the weight of the roof. Many glowing crystals were embedded in the walls, providing light for the prisoners.
Old Ba Sing Se
The old city was composed primarily out of rock houses; some of them were merely 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 smaller holes functioned as windows. There was no sign of other decoration. Many stairs led up from the paved floor to the numerous houses that were built into the wall.
A river ran through the center of the large cave that served as the city's main plaza. The ancient earthbenders had used their bending to shape the riverbed into neat channels in order 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 end.
Large stone columns stood at regular intervals to support the heavy roof and prevent it from caving in. The light of the city was provided by the naturally luminescent crystals that were spread all over the walls and floor.
- From older Avatar: The Last Airbender official site, originally on Nick.com. Encyclopedia now broken, archived at The Lost Lore of Avatar Aang - Location: Crystal Catacombs.
- Ehasz, Aaron (writer) & DiMartino, Michael Dante (director). (December 1, 2006). "The Crossroads of Destiny". Avatar: The Last Airbender. Season 2. Episode 20. Nickelodeon.
- DiMartino, Michael Dante; Konietzko, Bryan; Varney, Janet & Faustino, David (March 10, 2015). "Beyond the Wilds" commentary. Book Four: Balance Blu-ray.
- Yee, F. C. (author), DiMartino, Michael Dante (author). (July 16, 2019). Chapter One, "The Test". The Rise of Kyoshi. Amulet Books.