[TheStorm]

Put some smoke or something in the sewer, (look among emitters i think). Adding the worldspawn fog will affect the entire map. Remember that effects like smoke, fire etc reduces fps.

Try D-Day smoke (think there are two of them, one really thick), but the D-Day smoke is quite large so it might turn up a little bit above the sewers to. You'll have to try some different thing until you are satisfied.