Engineers are building the first bridge over a moving rock glacier

A grizzly bear explodes out of the shadowy brush in front of me and bounds up the mountainside, pausing to look back for one soul-shaking moment before continuing over the sunlit ridge with her two cubs.

Polychrome Pass in Denali National Park, Alaska, is full of such sublime sights. Streams the colour of pencil lead crash down from rapidly disappearing glaciers into the green tundra. Golden eagles glide over vast mountains of pink, brown and yellow volcanic rock. Far to the west, clouds …