The Underground (地下街 Chikagai?) 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The Underground city was originally made to be a safe place and expansion of land for the Walled world, as they could not expand outward.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 peoples 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.
Before the Fall of Wall Maria, Levi, Furlan Church, and Isabel Magnolia lived int he underground and worked as thugs to make ends meet. However they were one day offered citizenship im 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 plans and refuses to enter the tunnel, alerting the Survey Corps 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.