City 17 Underground

The City 17 Underground is a series of underground tunnels, parking lots and storage areas. Following the Combine invasion and the Seven Hour War, the City 17 underground was no longer used and fell into serious disrepair, however, it is possible that Resistance members used the underground even before the rebellion, as there are numerous bodies to be found by the player, only some of them being recently deceased.

After the destabilization of the Citadel, the underground became severely infested with headcrabs and headcrab zombies, as well as Antlions, who, following the collapse of the Combine's defensive field (previously powered by the Citadel), started establishing a foothold in the city via the tunnels.

Because of the large number of Zombines in the underground, it is possible that the Combine were attacking resistance members hiding there when Antlions and Xen wildlife were allowed access.

Features
The City 17 underground mostly consists of underground parking lots, maintenance tunnels, and storage areas. All or most of these facilities are in serious disrepair and many are infested with Zombies and Xen wildlife. There are many storage rooms and lots, as well as sewage pipes. The passageways connecting other areas contain handwheels to open gates.

Half Life 2: Episode One
After stabilizing the Citadel long enough to make 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, swarms of Antlions, and overcoming various other obstacles, Gordon and Alyx were able to locate an elevator, which they managed to power up and ride to the surface, escaping the hordes of zombies.

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