I know this is a bit of an odd question, but I ponder about these things sometimes. Do sewers (i.e. the domestic waste-water pipes under the road, that houses are connected to) run on gravity, or is there lots of pumping machinery under there?
The reason I ask is that I was looking at a big new housing development not far from me, in quite a hilly area, and trying to work out the logistics of managing all the waste-water and directing it off the site. It seems pretty complicated.
And what if a new housing development is built at the bottom end of a town, at a lower elevation than the main sewer? Do they have to build a whole new sewer lower down, or do they pump it up the hill, or what?