Most players rush through the initial sewer level, fighting off the standard green mutants. However, early in the level, after you drop down into the main water channel, look at the minimap. You will see a thin grey line leading off to the left that seems to go nowhere.
Follow the left wall until you see a large, dark pipe that isn't spewing toxic gas. Unlike the other pipes, this one has a slightly lighter texture at the base. Walk into the pipe. The camera will shift slightly, indicating a loading zone.
Ignore the obvious door leading forward. Instead, look at the shattered stained-glass window depicting a saint. Normal windows are just textures. This one, however, has a visible crack that you can walk through. Use a grenade or rocket launcher to blow the remaining glass away.