Palcacocha - the Quechua name roughly means forked lake - sits in the Cordillera Blanca at 4,566 meters, tucked into the Cojup valley below two giants: Palcaraju at 6,274 meters and Pucaranra at 6,156 meters. The glaciers that cloak these peaks feed the lake. Meltwater fills it, and a moraine - a natural dam of glacial debris - holds the water against its slope. Twenty-three kilometers to the southwest, in the Callejón de Huaylas, lies Huaraz, population roughly 100,000. The lake has supplied the city's water for generations. It has also, once, and possibly again, threatened to destroy it. What matters about Palcacocha is the specific geometry of the glacier, the lake, and the city directly downhill from both.

December 13, 1941 was an early morning. Most of Huaraz was asleep or just waking. High above the town, without warning, a huge piece of the adjacent glacier - or possibly an avalanche, the chroniclers are not entirely sure - fell into Lake Palcacocha. The impact generated a wave. The wave broke the moraine. Ten million cubic meters of water began rushing down the Cojup valley, gathering rock, soil, and ice as it went. It destroyed Lake Jiracocha further down the valley, adding that water to the flow. Fifteen minutes after the glacier fell, the mudslide reached Huaraz. Four hundred cubic meters of debris buried parts of the city. The death toll has been estimated at 1,800 at the low end, 7,000 at the high. The uncertainty exists because the city did not have time to count its missing before the mud set.

After the disaster, engineers installed drainage structures in 1974 designed to keep the lake level eight meters below the height of the damming moraine - a buffer against a repeat event. It worked, in 1974's terms. But the adjacent glacier kept retreating. Climate-driven melting accelerated through the decades, and the lake kept growing. By 2009, its volume had reached 17 million cubic meters. A University of Texas at Austin study found that by recent estimates the lake was 34 times larger than in 1970. At the same time, Huaraz itself has grown from 25,000 people in 1941 to around 100,000, many now living on the exact terrain the 1941 flood covered. The hazard geometry is the same. The population exposed to it is four times larger, and the trigger - the glacier - is being destabilized by warming every year.

In April 2003, NASA scientists spotted what looked like a crack in the glacier above Palcacocha on Terra satellite images from November 2001. Their warning reached Peru about two weeks after UGRH, Peru's glaciology unit, had completed field mapping at the lake. Panic spread through Huaraz. Innsbruck University researchers later concluded the NASA warning had been a misinterpretation of satellite data. That did not change the underlying problem. In 2010, UGRH presented a plan to drop the water level by 15 meters. The national government shifted responsibility for glacier lakes to regional authorities, who could not find the funding. In 2011 a stopgap measure installed six drainpipes, which lowered the water level by three meters by July 2013. Three meters, against a lake that has grown 34 times.

Saúl Luciano Lliuya is a farmer and mountain guide from Huaraz. In 2015, with support from the NGO Germanwatch, he filed a lawsuit against the German energy utility RWE in a German court. His argument: RWE is responsible for roughly 0.47 percent of global anthropogenic emissions since industrialization. That small percentage should translate to 0.47 percent of the cost of flood protection measures at Lake Palcacocha - about 17,000 euros of the estimated cost. RWE called it absurd. The District Court in Essen dismissed it. Luciano Lliuya appealed to the Higher Regional Court of Hamm, and in 2017 that court did something no court had done before: it ruled that the suit could proceed to evidentiary stage, establishing that a private company can, in principle, be held partly responsible for climate damages. The court sent experts to evaluate the flood risk of Palcacocha itself. A farmer from the Andes had made a German court look at a Peruvian glacier and ask how much it had melted - and whose fault that was.