Seven kilometres of climbing in the Cordillera Blanca of the Peruvian Andes end at Laguna 69, a lake of electric turquoise at 4,604 metres. A glacier upstream grinds stone to a powder fine enough to stay suspended in the meltwater, and that powder throws the light back as turquoise. The lake sits in a cirque, a rounded hollow the ice scooped out of the mountain and then retreated from, dammed behind the rubble it left as it went. The walk up starts at Cebollapampa and takes you about seven hundred metres higher. The first forty minutes run level along the valley floor, through open Andean meadow purple with lupine and dotted with low, twisted trees. Those are queñua, with peeling papery red bark, and they grow higher than almost any tree on Earth. Then the grass gives out and the trail switchbacks over fields of shattered rock, the footing loose and every step at this height leaving you short of breath.
The drive to the trailhead passes Yungay. In 1970 an earthquake shook a slab of ice and rock off Huascarán and sent it down this valley, burying the town in minutes. A new one was built nearby, and the old site is the memorial garden you drive past on the way up. At the top the ground drops away and the lake is simply there, with the rock-and-ice walls of Chacraraju rising straight out of the water. A thin waterfall comes down the headwall opposite you to feed it. People sit on the boulders with their lunch and their breath short, and on a still morning the snow faces lie doubled on the surface. Peru holds most of the world's tropical glaciers and has lost well over half of them in six decades, and lakes like this one are what the meltwater makes on the way down. At the edge where the melt runs in, the colour is strongest of all, because that is where the rock flour is arriving, straight off ice that is going.