A deadly Swiss avalanche vividly demonstrates that water-saturated soils can increase the danger posed by unstable mountains.

In August 2017, 3 million cubic metres of rock broke off the face of Pizzo Cengalo, a mountain in the Swiss Alps, and tumbled downhill. Thirty seconds after the rock avalanche slowed to a halt, its debris began moving again. The flow killed eight hikers and buried parts of a village.

Fabian Walter at the Swiss Federal Institute of Technology in Zurich and his colleagues analysed images and videos of the avalanche and visited the site to deduce what happened. They found that melt water from a nearby glacier had saturated the mountain’s sediments. Glacial ice eroded by the avalanche also became incorporated into the material barrelling downhill.

When the avalanche stopped, water from the melted glacial ice trapped inside helped to mobilize it again, sending another debris flow racing downwards. A similar avalanche on Pizzo Cengalo in 2011 didn’t trigger deadly debris flows, probably because the sediments were dry.

Emergency managers should keep in mind that waterlogged soils can amplify the damage caused by a mountain avalanche.