The Underground is a mysterious faction consisting of sewer-dwellers who live underneath Washington D.C. Nothing is known of their history or their intentions, although they appear to be scavengers who come up to the surface to gather supplies. They are hostile to anybody they encounter. Unusually, Underground enemies have no voice lines whatsoever, and are only audible through their heavy footsteps.
Encounters with the Underground are infrequent and mostly randomized, although they are most commonly found in D.C.'s sewer system. They make a scripted appearance in the side missions Beekeeper Jeff, and Navy Hill Transmission.
Visually, the Underground are identifiable by their all-black clothing and the bright yellow flashlights worn around their heads. The Underground Tank wears heavy armor made from large chains.
Underground units are armed with basic weaponry limited to semi-automatic rifles, shotguns, and knives. Their Throwers use special grenades that apply chemical damage to their victims. Their Tanks are armed with chainsaws and behave similarly to the Outcast Tank.
- Assault - Basic infantry unit armed with semi-automatic rifles. They have no unique abilities.
- Rusher - Agile units armed with knives. They can throw their knives from a distance or rush their target with melee attacks.
- Thrower - Special units armed with chemical grenades that apply effect damage to their targets. They are armed with pump-action shotguns.
- Tank - Armored unit equipped with rudimentary armor and a chainsaw. Tanks have no ranged weapons and can only use melee attacks; their chainsaws can apply the bleeding effect to their targets.