Definitely an off-the-beaten track experience, the Sewer Museum (or 'Musée des Égouts) at Porte d'Anderlecht gives you a glimpse of underground Brussels. Yes, you can take that literally, as you'll descend under the streets into a part of the working sewer system, and you'll get to see the River Senne that today flows under the city.
The entrance of this unusual museum is situated in the Pavillon de l'Octroi, one of the two former toll pavilions at the Porte d'Anderlecht that used to collect taxes on goods that entered the city. It costs 8 euro to visit, but every first Saturday of the month a free guided tour, led by experienced guides of ex-workers, is offered. It might be wise to plan your visit accordingly.
The first exhibition spaces show maps of old Brussels that explain how the sewer system was made, and how the river Senne was put below the city. You'll understand why at times there are so many mosquitoes in the metro station Bourse or Anneessens (the sewer runs underneath), and you'll also learn something about the underground wildlife of Brussels. Meet our community of Brussels rats! Luckily, no alligators have been found so far, but apparently that's daily reality in America though.
What I also like about the museum, is the aspect of sensibilation, where they point out that an average citizen consumes 150 liters of water per day (!!). All this 'dirty' waste water ends up in the same sewer as our 'clean' rain water, all of which then has to be processed by water treatment plants. Brussels hasn't got a smart dual sewer system as they do in newer cities such as Louvain-La-Neuve unfortunately.
When you descend to the lower exhibition spaces, you'll smell you're getting closer to the sewer. (I hope you brought your clothes-peg?) Still, for us, entering the bowels of Brussels was the most exciting part of our visit! We walked along a part of the river Senne, and entered one of the tunnels of the sewerage network. The entire sewer network under the City of Brussels is nearly 400 km long, and in the museum you'll get to walk about 300 meters underground.
At the end of the obscure tunnel you're in for a treat. The Brussels graffiti artist PAROLE has covered the walls of this waste water collector with his calligraphy art. If you look well, you'll recognise a series of words that tell the story of sewer workers. The work is ephemeral as floods will gradually carry the message through the bowels of the city.
You'll exit the sewer on the other side of the street, where the last exhibition spaces tell you more about the life of a sewer worker. Finally, you will leave the museum through a souvenir shop in the second toll pavillion (feel free to breathe again.)