"A few meters away (note: after the 318 m long 19th tunnel) begins the 20th tunnel, which also rises in a spectacular spiral through a rock tower to almost below its summit and exits there. The center of the rock tower, around which the three tunnel spirals rotate, is surrounded by a series of mine holes cut horizontally into the rock. The holes, made at regular intervals, could each hold two boxes of blasting gelatine, which could have been used to blow up the entire rock tower if necessary and thus interrupt the road.
Due to the lack of fresh air supply, drilling with drills and explosives proved to be extremely difficult, especially in the long tunnels. Acetylene lamps were usually used during excavation work." Source: Information sign on the side of the road.
This 86 m long spiral tunnel, which winds upwards through the rock in a 1080 degree spiral, is certainly one of the highlights of this hike. For this tunnel you definitely need a headlamp, if necessary a bicycle lamp or enough battery on your smartphone.