The best reason to come to Witten. This walking trail runs through the Muttental valley south of the city, passing a series of preserved and reconstructed mine sites spanning the region's coal-mining history from medieval drift mines to 19th-century pit-head buildings. The trail is clearly waymarked, about 6km in length, and can be walked in two to three hours at a comfortable pace.
The open-air museum aspect works well: the mine shafts and buildings are set in a wooded valley with the Muttental stream running alongside, making it a genuinely pleasant walk rather than a trudge through industrial heritage. The oldest surviving mine on the trail dates to the early 19th century; the geological outcrops along the way show the coal seams that made the Ruhr the industrial heartland it became.