I'm not sure if this is a server or a game engine bug, but this is what I see when I go in the sewers:
(http://img819.imageshack.us/img819/7189/sewerbug.gif)
There are just certain parts of the sewers that visually do not load. Once I take about 5 steps into the pitch blackness, however, they appear.
Happens for everyone AFAIk, don't know why.
walk into it and it loads.