The Canal Saint-Denis, just a bit past its junction with the Canal Saint-Martin and the Canal de l'Ourcq in northeast Paris, appears to flow downhill to pass under a bridge. It so much appears to that I had to stand there for a few minutes to remind myself that it couldn't be.
In fact, the walls to the left and right are lower and lower as they approach the bridge (see picture below from outside). Added to that, the arch and rise of the bridge...
The Saint-Denis and Saint-Martin canals were both built in the throes of the Industrial Revolution; their role was to provide transportation for the industries that grew along their banks. Even today, they serve as a bypass around Paris when the Seine is flowing too high.
Today, the Canal Saint-Martin has become the center of a BoBo (bourgeois bohemian) area along its lower stretches, while the Canal Saint-Denis is still lined with its past...but change seems to be coming.