Aside from the Eiffel Tower, the Louvre and Notre Dame Cathedral, Paris, often called the City of Romance, is also home to the Musee des Egouts de Paris, or Paris Sewer Museum. Museum visitors can tour part of Paris' extensive underground sewer system. This stretch, fortunately, is protected from raw sewage, so the smell isn't too bad. The museum, located within walking distance of the Eiffel Tower, also includes displays about the past and present of Paris' sewage system, which dates back to the 1200s.
More info: http://en.parismuseumpass.com/musee-musee-des-egouts-de-paris-23.htm