The Underground is a Praetorian zone much like the Sewer Network of Paragon City, with three interconnected "sub zones" that are "beneath" the cities of Nova Praetoria (Underground Nova), Imperial City (Underground Imperial), and Neutropolis (Underground Neutropolis). The Underground is a "safe" haven for The Resistance, although it is infested with Ghouls and the life down there, such as it is, is much less care-free than the life above-ground.
(Copied from the City of Heroes Going Rogue official web site)
The Underground is a network of tunnels that traverse the soft underbelly of Praetoria. It was created to keep the appearance of the above ground portion of the city unspoiled and uncluttered while the day-to-day maintenance and transportation tasks were carried out away from public view. Using the Lethe as its main artery, the tunnels follow a winding path through Nova Praetoria to Neutropolis and beyond.
Because the Underground is situated away from the prying eyes of the local citizenry, it is an ideal meeting place, hangout, or headquarters for groups that prefer to do their business covertly. It is more difficult for beings to perceive threats while in the Underground due to a well-known perception filter anomaly of the area, making the underground environment excellent terrain in which to hold secret deals and an environment full of prime opportunities for surprise attacks.
Despite the Underground's primary function, which can range from the mundane (transport of products and commuter passengers or elimination of waste materials) to volatile (transport of highly confidential personnel or material), it is surprisingly clean. In fact, the water conduits and large metal tubes located along the walls and housed away in storage rooms push clean, filtered water through the miles of pristine pipes.
Make no mistake, however, though the dark passages are clean, they provide no rest for the lost or weak. The tunnels are a dangerous place. This maze of winding halls and rooms is home to a strange breed of zombie-like creatures called the Ghoul. Ghouls prefer the dark spaces and freedom these tunnels provide, and they take flight from any scientific personnel who have attempted to study them.
The Underground is littered with gangs looking to keep their treasonous scheming away from Emperor Cole's network of spies. The Resistance, in particular, has set up several hubs across the Underground by hacking into some maintenance rooms and resetting the door codes to a series of specific Resistance-only logins, which are updated hourly. In fact, given the frustrating login changes to these hubs and a few access doors, Cole's city maintenance staff has reprogrammed the logins of many doors leading to the surface with loyalist codes of its own.
The illegal activity in the Underground, from smuggling to shakedowns to purely animalistic rampages from the Ghouls, is frowned upon by the Cole administration, so Cole has sent in several task forces of Praetoria police officers to investigate and monitor any and all gang activity, real or imagined.
There is little camaraderie in the tunnels. The rules of human decency, etiquette, and restraint shown under the light of day or cover of above ground night are absent in the Underground. In this vast vista of poor visibility and unending miles of concrete and tracks, behavior has eroded to the point where "ally," "enemy," and "meal ticket" have become nearly synonymous. While the Resistance, the Ghouls, and even the Destroyers may stick up for their own kind on the whole, every dark corridor provides an opportunity for bloodshed. Once the swinging starts, the only rule that matters is that the last one standing wins.