This location is a [grim] [prison-city] that [stands as humanity's first line of defense].
The Surface
This location is a [claustrophobic] [city] where [humanity built up instead of out].
The Underworld
This location is a [collapsing] [maze] that [contains dangers and the city's infrastructure].
The Deep Roots
This location is a [hidden] [cavern system] where [the purified wellsprings are protected].