A traditional approach when pedaling from point A to point B and there is a mountain in the middle of the path is to create a road to save it, commonly by zigzagging. Some bicycles have gears to overcome high slopes, but some others do not. In Bergen, Norway, they have built a tunnel between two neighbors of the same city (Fyllingsdalen and Mindemyren). Originally, one should pedal for 35 minutes, whereas today a cyclist can spend 10 minutes to arrive to the same destiny. This almost 3 kilometers long, 6-meter width tunnel is opened for cyclists (3.5 meters of bike line) and pedestrians. (2.5 meters). It goes by in parallel to the train line and takes advantage of the auxiliary tunnel originally built as a complement to the railway.
Apart from the obvious improvement in the sustainable mobility, it also opt for security because of all the cameras which register images 24-7. However, the tunnel is only opened from 5:30 to 23:30. Additional improvements over common tunnels for bicycles are the emergency phones and speakers throughout it. In case of emergency, ambulances and firefighters can access without problem.
The main problem architects faced when building it was the high humidity since the mountain is plenty of water and needed a drainage system to overcome it.
As far as I know, this is the second larger tunnel for bikes. The most large is called Snoqualmie in Seattle with 3.6 kilometers.