One hazard that results from mass wasting is slope failure. This can occur when a large mass of rock detaches from a mountainside and slides down the slope. Such a collapse is often sudden and violent, causing a great mass of loose debris to cascade downhill. In other cases, an entire layer of the ground near a slope can detach and begin sliding down a mountainside. Due to heavy rainfall in Japan in 1999, thirty-one people were killed as a slope collapsed.