The Underground River--Near Pont de l'Alma, Paris
When in Paris, visit the sewers. Forget Avenue des Champs-Élysées, the Eiffel Tower, or the Arc de Triomphe, one can take a self-guided tour a few meters below the streets in the sewer system of Paris. I must shamefully admit, this was the only tour I had set out on my agenda for my weekend after hearing about the history of the tours in a historical geography module. In the 19th century, Paris elite would take river boat tours of the newly constructed system as part of a Sunday outing! Once you've made the descent below the street, the tunnels open up to a walking height and you are free to roam a section of the system. Periodically, one must duck to dodge large dripping pipes (let's hope they're dripping clean water...) Below, under a grated cage walkway, runs the open sewage, and it is truly sewage, toilet paper and all. Aside from the odor, which is not a deterring factor from the tour--it really isn't noticeable--the tour gives a nice historical and modern representation of the system, which any engineer or curious individual would find fascinating.