The world is full of narrow things
The cave is called the tunnel that was built in 1786 for a water-pumping works that would fill a landscaped small lake on top of the so-called Cave Hill. The water would be taken from Brunnsviken via a vertical shaft, which blew up simultaneously. The pump shaft is visible as a round pits on the mountain. The vertical shaft and the horizontal tunnel never met, because the work was interrupted shortly after Gustav III’s death.
