Two huge boulders sent tumbling by a landslide narrowly missed a farmhouse in Ronchi di Termeno in northern Italy on January 21, 2014. The above photo, taken two days later, shows one of the boulders after it rumbled down the hill and destroyed the barn before coming to rest in the vineyard—halted within a meter of the house.

The second boulder, hidden behind the house, stopped just short of the building. The boulder in the foreground is from an older landslide. Members of the Trebo family, who live on the farm, were reportedly unharmed.

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These boulders appear to have originated from a large pillar of rock, probably limestone, says Ben Mackey, a geologist at the University of Canterbury in Christchurch, New Zealand. Erosion can cause pillars to slowly detach from the side of a cliff, then collapse, sending boulders tumbling down.

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It's unclear what triggered this rockfall, geologist Mackey says. But such rockfalls can be caused by storms or earthquakes. "Sometimes, though, rocks will fall with no apparent cause," he says. "Whatever was holding them in place just lets go. This makes predicting when rockfalls will occur quite difficult."

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While smaller boulders tumble down cliffs often, Mackey says, huge rockfalls like this one are fairly rare. In a given location, boulders of this size would fall maybe once in many thousands of years. Mackey studies the old boulders of Christchurch to try and determine when they toppled over and how often such rockfalls may occur.

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