Underground (Anime)

The Underground (地下街) 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.

A Choice with No Regrets: Part One
Levi, Furlan Church, and Isabel Magnolia are captured and blackmailed to join the Scout Regiment by Erwin Smith.

The Female Titan arc
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 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.