I know it's a big use of valuable prims but it might be quite nice to have a part of CD with sewers beneath it?

Given that a lot of the surface is already covered with prim road/buildings I think that it would be just a matter of digging a trench in the terrain below them and then putting in some pipes and manholes. If the pipes run below the water level then they also appear quite gloomy.

I'm not suggesting that the entire sim be done but a small network with some manholes/access points in "interesting" locations?