The Passo Mortirolo is one of the hardest cycling climbs there is. It’s around 11.5km and averages nearly 11% to the 1,852m summit. The maximum gradient is 18%.
The road started life as a goat track, and it’s not that much wider now. You get glimpses back over the Valtellina valley, but otherwise it’s pretty much 11km of interminable 10+% gradients on a narrow single lane road that winds through dense, airless woodland. Every so often, the woodland falls away and the road breaks into open pasture - this delivers some sort of relief, even though the gradients don’t ease off at these points!