Iskander:
urban survival works, to locate manholes and grates and such, above and below ground, right?
how does that skill work in northport?
Also just because you know of a sewer entrance also doesn't mean you know of a way into the Under City, or where the sewers go, how they connect, what directions they run, or what dangers lurk (that would be Area Knowledge (UnderCity) or Area Knowledge (Sewers)).
You could certainly (upon finding yourself in a sewer) use Urban Survival to to recognize a dangerous bit of construction (loose brick work, an arch soon to collapse, etc), a section that floods frequently, and maybe if a dangerous sewer fauna or flora lairs there (maybe?).
I base this on what the rules for Urban Survival say, not actually what our Narrator has said as I never asked about Urban Survival.
However based on what little I've been told about the megadungeon that is the Undercity... I suspect Urban Survival to be of some use down there (what I mentioned above), but the real useful info skill is Hidden Lore (UnderCity)*.
* As well as Archaeology (UnderCity), Architecture (UnderCity), Area Knowledge (UnderCity), History (UnderCity), Navigation (UnderCity), and Survival (UnderCity)**.
** It takes a Perk*** to gain access to the UnderCity specialization, Jareth has 3 such perks (to make sure I could cover all those skills and because two of the perks are Contacts (Karlow and Aethebert), and the third gives him the Keys to the Sewer doors).
*** Contact: either Karlow, or Aethebert; or Civil Servant (Courtesy Rank) Sanitation and Sewer Worker (I know of those three for sure). Also I think the "Lives In The UnderCity" Perk is available with the right background and race.
This message was lightly edited by the player at 14:17, Tue 07 June 2016.