Nearly a decade ago, a helicopter pilot flying in Canada’s Northwest Territories noticed something peculiar: a landslide so immense, it had blocked a river and created a lake . But that damming power wasn’t what made this slide so strange. What he saw wasn’t just new to the landscape, it was new to science, what researchers have dubbed a “permafrost detachment slide.”

In a recent paper , a team of scientists describes two of these events in the region, one stretching for three-quarters of a mile and the other for 2 miles. Combined, they’d moved 30 million cubic yards of earth. Whereas other kinds of slides send a torrent of rainwater and mud and rock down a hillside, permafrost detachments happen when frozen soils degrade and the land fails catastrophically.…