Underground

The  is a subterranean city beneath the capital city of Mitras. Similar underground cities were planned for towns such as Stohess District, but were abandoned and fell into ruin.

Story
At some unspecified point in time, Kenny Ackerman discovers that his sister Kuchel Ackerman 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 Ackerman, 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 Survey Corps.

The Female Titan arc
As part of the plan to capture the Female Titan in Stohess District, Armin Arlert leads Annie Leonhart, escorting Eren Yeager with Mikasa Ackerman, to a tunnel leading down into an abandoned site for another underground city. Annie 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.