Bryan and Lisa talked me into taking a sewer tour of Paris -- while it was stinky, it was interesting. Did you know that Paris started charging folks to view the sewers in the late 1800s. At one time you could actually ride in a flat boat along the sewer in the sewage, as part of the tour.
Now it is much more mundane, but there is a passage through some narrow chambers that is a little nerve wracking. In several places, especially on a rainy day (like when we went) there is some serious water pounding through there. I would not recommend it on a late afternoon in August. Even on a cool day, with lots of rain there was a stench that was pretty strong. Take a flashlight and a scented handkerchief. Don't forget your boots!
The sewer department has done a nice job of detailing and documenting the history in french and in english! So you get a good scent of what is going on down there.
There was even a rescue...
and of course there is the evidence that humans can't pick up after themselves...