The Underground (地下 Chika?) is a subterranean city beneath the innermost land in the capital city. Similar underground cities were planned for towns such as Stohess District, but were abandoned and fell into ruin.
The Underground city is one of, if not the poorest place within the Walls. It is a crime ridden city, filled with criminals and homeless people. There are very few jobs in there, so a lot of people go into the criminal world to earn some money. Due to it being underground, the light from the sun does not reach the city, so the city is constantly illuminated.
The Underground city was originally made to be a safe place and expansion of land for the Walled world, as they could not expand outward. However, there were many downsides to people living underground, including the fact that a lack of sun caused people's legs to become unusable and poorly working. Because of this, the lesser population was forced to live in the underground, and not allowed to come to the surface without citizenship. The rest of the walled population abandoned the idea of underground living and left it to the more impoverished people.
At some unspecified point in time, Kenny Ackermann discovers that his sister Kuchel Ackermann has become a prostitute in a brothel located in the Underground and that one of her clients has impregnated her. Some time later, he visits Kuchel at her brothel but discovers that she had become ill and died. Her son, Levi Ackermann, is all she left behind before her passing. Kenny takes it upon himself to raise Levi in the Underground, training him in knife-wielding combat and teaching him social skills. After seeing Levi reach a point where he is able to fend for himself, Kenny abandons Levi, trusting that through his own abilities, he will some day leave the Underground. Levi later grows up to become a notorious thug in the Underground before meeting Erwin Smith, who convinces him to join the Scout Regiment.
Before the Fall of Wall Maria, Levi Ackermann, Furlan Church, and Isabel Magnolia lived in the underground and worked as thugs to make ends meet. However they were one day offered citizenship in exchange for pretending to join the Scout Regiment and killing Erwin Smith. After a coordinated stealing of goods, the three are chased by soldiers of the Scout Regiment and are willingly captured. Squad Leader Erwin Smith then blackmails them into joining the regiment so he can make use of their skills.
As part of the plan to capture the Female Titan in Stohess District, Armin Arlelt leads Annie Leonhart, escorting Eren Jaeger with Mikasa Ackermann, to a tunnel leading down into an abandoned site for the underground city, hoping that she could be restrained and confronted down below so that she would be unable to transform into a Titan and run away. Annie is aware of their plan and refuses to enter the tunnel, alerting the Scouts to her deception. As she is ambushed and restrained, Annie transforms into the Female Titan, putting an end to the underground plan of the operation. Much of the underground tunnel is later destroyed by Annie in an attempt to locate and kidnap Eren. To do this, she stomped her Titan's feet through the ground and made the tunnel visible from the surface, ruining most of it in the process.
Refugees from Wall Rose had been sent to the underground city at the outbreak of the Wall Rose invasion. However, food supplies only guarantee proper treatment for the refugees for one week before the risk of civil conflict began. To prevent this, the military declares Wall Rose Titan-free one week after the start of the outbreak.
With Captain Levi's backing, Queen Historia would gather many orphans and people in need from the Underground then take them to a farm funded by the Royal Family's budget and the assets seized from politicians.