It doesn't actually smell that bad. It does smell of sewer, but sewer smell not the same as being in a porta-potty that hadn't been cleaned for a week, or a music festival urinal. It's more of a musty cave smell. Organic?
Paris's overarching narrative is much the same as London's: Things got crowded, disease spread, sewers were constructed.
I'm writing this post in a bit of a hurry (hopefully will get a chance to add more later) so for now I'll let the photos do the talking!