City 17 Underground

The City 17 Underground is a series of underground tunnels and parking lots. Following the Combine invasion and the 7 Hour War, the City 17 underground was no longer used, and fell into serious disrepair, however, it is evident that resistance soldiers possibly used the underground, as there are visible bodies littered around following the destabilization of the Citadel, which were recently deceased.

After the destabilization of the Citadel, the underground became severely infested with headcrabs and headcrab zombies, as well as Antlions.

Features
The City 17 underground looks as if it was used as an underground parking lot and storage prior to the Combine invasion. There are many storage rooms and lots, as well as sewage. The passageways connecting other areas use handwheels to open gates.

Because of the underground's dark and abandoned atmosphere, it became heavily infested with Headcrabs, and, after the destabilization of the Citadel, it had also become infested with Antlions. Headcrab zombies are abundant in this area.

Half Life 2: Episode One
After trying to stabilize the Citadel long enough for their escape, Gordon Freeman and Alyx Vance became trapped in the underground after the razor train they were riding in derailed and crashed. After battling hordes of zombies, Antlions, and braving other obstacles, Gordon and Alyx were able to locate an elevator, which they were able to get into and escape through before being killed by the hordes of zombies.

The City 17 Underground is featured in Half Life 2: Episode One's Lowlife chapter.