The Paris Urban Adventures (PAU) group took a little trip down below to check out the Paris sewers. While on the lookout for R.O.U.S.s and Ratatouille, in the depths of the city we did manage to learn a whole host of new things about how the Sewers of Paris work.
What is true is that most of us never think about where things go when we wash them down our sinks, or flush them down our toilets, or lose them in the city street sewage drains. The tour is well worth taking, and full of interesting facts and tidbits.
The most interesting fact I learned was that the methods designed to run and clean the sewers 150 years ago are still used today, and you get to view many of these devices while on the tour. 
What ever you do, DONT ask about the strangest things they've found while clearning the sewage...you don't want to know what people have flushed down. What did make me laugh, though, was at the end of the tour they direct you to their boutique. "Boutique" I asked? What could they possibly sell? Long lost wedding rings found while cleaning the sewers? Gold-teeth accidently spit out? No. Nothing so exciting I'm afraid. They sell key-chains, little puzzles, stuffed rats and even base-ball caps with the Paris Egouts logo proudly displayed. The stuffed rat wasn't even Ratatouille, which may have been worth the coins if it had been. I would be curious what their annual intake on these items are.
Where: The tour starts at the Pont de l'alma in Paris, which is just east of the Eiffel Tower, on the same side of the Seine. You won't miss the sign, it says, "Le Musee de l'Egout", and Egout means sewer in French.
How much: It costs just over 4 euros per person for a 1 hour tour. Some tours are apparently in English, though we were not able to obtain this on our tour day.
Photos: To see some photos from this tour you can check out the PAU photo gallery here.