Musée des Égouts de Paris – the Paris sewers
This great and unusual attraction was one of my favorites in Paris because it allows you to see things you normally can’t. It is a fun departure from the normal Paris attraction circuit.
First, there is a museum portion. This has information showing the changes in sanitation history from Roman times to now. Further, it shows 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 particularly nice, 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.