Lake Palcacocha is high in the Cordillera Blanca range of the Peruvian Andes, sitting above the city of Huaraz at an altitude of about 4,500 metres. When the lake broke through the extensive moraines, or natural dams, holding it in place on 13 December 1941, it sent nearly 10m cubic metres of water and debris into the narrow valley towards the city, 1,500 metres below. The result was one of the most devastating glacial lake outburst floods – or “GLOFs” – ever recorded. The force of the water altered the area’s geography for ever, and killed at least 1,800 people, and possibly as many as 5,000. Like all such lakes, Palcacocha was formed as a glacier receded, the water filling up the hollowed-out land around it. This process – and the floods that can result – is natural but now, scientists say, the climate crisis is increasing the risk it poses. Peruvians stand on the site of a hotel in Huaraz, which was destroyed in December 1941 when flood waters from Lake Palcacocha, carrying thousands …