In June, Canadian officials warned that a nameless lake in the Northwest Territories was about to fall off a cliff. It did. On July 15, a section of ground ice that's been around since the last ice age gave way, sending roughly half of the lake's 1 million cubic feet of water—the equivalent of a dozen Olympic-sized swimming pools—cascading into a valley below in a five-story waterfall that lasted two hours. "Water rushed over the slump debris and down a narrow valley before emptying into a larger lake" about 3 miles downstream, leaving the nearby community of Fort McPherson unscathed, according to the Northwest Territories Geological Survey. A time-lapse video shows the collapse along with an ooze of mud and water.
The 1.5-hectare lake had been resting on the cliff held by icy headwalls, preserved by permafrost for millennia. However, wind and rain exposed the headwalls over time, causing them to melt, per the Canadian Press. "It was one of those things that you can get out of the way of but you can't stop," an official says. Though only half the lake drained, the government says the rest might still collapse as newly exposed permafrost starts to melt. While these so-called thaw slumps—basically the Arctic's version of landslides—have been occurring for millennia, they "are more abundant and much larger than they were in the recent past," thanks to warming temperatures and increased rainfall, per the NTGS. (Here's why glacial lakes are vanishing.)