sewer or sewers
Solution 1:
We do not say "the sewer of Paris", unless we mean "the sewer that is Paris". We say "the sewers of Paris". A charming rat who was a cook found himself in the sewers of Paris.
It may be regional, but in practice in American "sewers" refers to a sewer system and "sewer" either refers to that part of the sewer system at hand ("Oh man, my phone fell through the grate into the sewer."), or is used as what amounts to a euphemism for a pungent soup of fecal matter and urine. "Get your mind out of the sewer."