In 2018, a strange and enormous landslide occurred in Canada’s Northwest Territories. A slab of earth roughly equivalent in volume to seven Great Pyramids of Giza suddenly began sliding downhill, leaving behind a barren scar in the forested landscape. But what puzzled scientists was that the slide had initiated in frozen ground that had been glued in place for years.

An analysis of the event reveals that it represents a new class of catastrophic landslides called permafrost detachment slides, researchers report in the Sept. 28 Geophysical Research Letters . The slides may be set in motion by wildfires that occurred decades earlier, destabilizing the ground and opening cracks through which water infiltrated deep underground.…