Most people don't know this exists — There's an actual museum in the sewers below the city near Pont de l'Alma. It sounds gross but it's actually clean and shows how Paris built one of the world's most advanced sewer system in the 1800s.

The history is wild — During Haussmann's renovation of Paris, they built 2,400km of sewers that are still functioning today. You walk through actual tunnels (not smelly ones) and see the engineering that made modern Paris possible.

Entry €4.40, open weekends and some weekdays. Takes about 45 minutes. It's genuinely educational and kids love the weirdness of it. Much more interesting than another church if you're into urban history.

Not for claustrophobic people — You're definitely underground in narrow spaces.