This great and unusual attraction is currently closed for renovations, but it was a fun departure from the normal Paris attraction circuit before, and I’m sure it will be again when the renovations are complete.

First, there is a museum portion with information from the Roman times to now and explaining how the sewers work, how they are cleaned (using things called pigs), etc.  And also stories about historical events that relate to the sewers.  After that, you can go down and see the actual sewers, which do not smell good as you would expect.  But the whole place should only take about an hour to go through and they present the info very well in the self-guided tour.