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 swirl around the 6200-metre summit of Denali, North America’s highest peak.

Visitors used to take in these sights via a gravel road cut into the steep slope 120 metres above the valley floor, a route so dizzying that some would leave the bus and walk. Then in 2014, a section of the road the length of a football pitch started collapsing down the mountainside.

“It was dropping several feet a year,” says Brad Ebel, a former maintenance crew leader. “It was obvious that it was speeding up.”

Read on at New Scientist