The Frank Slide was one of the largest & deadliest landslides in Canadian history, where 82 million tonnes (90 million tons) of limestone collapsed from Turtle Mountain & descended upon Frank, Alberta.  The town of Frank was founded a coal mining town on September 10, 1901.  At 4:10am on April 29, 1903, a section of the summit of Turtle Mountain about 1,000 metres across broke off,  travelling down the mountainside at a rate of 112km/hr (70mph) before cascading upwards on the opposite side.  Prior to that it had long been known by First Nations as "the mountain that moves", refusing to live anywhere near it.  There were also reports about constant rockfalls & slides.  About 100 people lived in the path of the slide, mostly on the outskirts, & it is estimated that about 70-90 lives were lost.  Most of their bodies remain entombed w/in the slide, about 12 skeletons were located in the immediate aftermath & six more were unearthed during the construction of the Crowsnest Highway in 1924.  About 17 miners who were working the coal seams under Turtle Mountain became trapped when the slide caved their main portal, & the slide had dammed the neaby river which caused water to start seeping through a caved second portal into the mine.  One miner knew the coal seam extended upwards to the surface so they worked up the coal seam until the toxic air started to weaken them.  By then three last miners continued digging until they broke through the surface, able to tunnel themselves out after 13 hours underground.  While the main part of the town of Frank was spared, it grew in size later on to the mining boom before slowing down to the current time.  Now today there is an interpretive center & trail as the slide remains largely unchanged.  You can see that some of the boulders here are massive.  About 80 monitoring stations have also been placed on Turtle Mountain as scientists are aware that another slide of smaller scale is likely imminent one day, perhaps from the south peak to the right of the slide in this view.  A few kilometres away is the Hillcrest Mine memorial, site of Canada's deadliest mining disaster in 1914.

More info here: https://en.wikipedia.org/wiki/Frank_Slide