Disclaimer: This is not an excerpt from a Neil Gaiman novel!
The canals under Vienna are more special than you'd think and much, much more special than most people know. Not only, because a good and functioning sewage system is vital for any city and its population's well-being.
First, for us history geeks: In 1739, Vienna was the very first European city to have an area-wide, extensive sewage system beneath its streets and edifices.
Second, for us movie buffs: The awesome movie The Third Man, set in a hardly recognizable Vienna destroyed by the Second World War, features the Vienna sewers as part of its star cast, with Holly Martins' and Orson Welles' chase set largely in the undercity. Today, you can do a tour of the city based on the movie, which also takes you into the extensive tunnel system under the streets - don't worry, it's not what most of us would expect!
I myself haven't done that tour yet, but I really want to. So wait until I make it back to Austria again and we can check it out together.
Idea and historic date: Unnützes WienWissen