That makes perfect sense, as this is the former drainage canal and city sewage system. Nowadays, you can take a tour here without getting dirty.
Walk through the largest and invisible national monument of Bergenop Zoom: the Grebbe, a former drainage canal and sewer
From drainage canal to city sewage system: how did this come into being? This question is answered when you take a walk through the Grebbe. It starts in the 13th century with the excavation of peat. To transport it, a drainage canal was dug straight through the city. Later, the waste was also discarded in the Grebbe and the first vaults over the canal appeared in the 15th century. This caused the Grebbe to lose its function as a peat canal and it became a sewer.