A shortcut through the Brunkebergsåsen ridge, which used to make east-west movement in Norrmalm annoying and steep. It was built so pedestrians, cyclists and originally also light horse traffic could avoid climbing over the ridge. What makes it interesting is that it is basically 19th-century micro-mobility infrastructure: not glamorous, but very practical. Stockholm had a topography problem, so it drilled a direct route through the obstacle. A nice extra detail: it was originally a paid tunnel, and many people kept using the street above because the tunnel cost two öre to enter.
