"Going below the city to visit the Paris Sewers Museum (Le Musée des Égouts de Paris) was a surprisingly fascinating, off-the-beaten-path experience: the museum lets you walk through part of the active sewers and even hear the waste flowing beneath you, but it actually doesn't smell that bad. The exhibit is very informative — most signs are in French, German, and English — and begins with pictures and old machines used to clean the system (look out for the large metal balls that were rolled through the sewers to unblock debris). The sewers opened to tourists during the 1867 World Expo, the self-guided tour takes about 45 minutes, and audio guides (English, French, Spanish) are available and recommended since a few displays are only in French. Practical details: it's near the Alma Marceau metro stop (right across the bridge on your left), open Tuesday–Sunday 10:00–17:00 with last admission at 16:00, admission is 9 EUR for adults with discounts and free entry with the Paris Museum Pass. I highly recommend the visit — it's unique, there's usually no line, and do bring hand sanitizer when you're done." - Matthew Kepnes