Avalanches are not triggered by noise, but they are the only natural hazard that may be triggered by individuals walking, skiing, snowmobiling, and snowboarding on unstable snow. Many avalanches occur naturally without human provocation. Avalanches are great masses of snow that slide down mountainsides. The snow may be powdery, sliding over compacted older snow; slabs of snow that roll down the slope; or a combination of ice and snow from the slope, including rocks and other debris. Dry slab avalanches are the most dangerous, traveling at speeds of 60–80 miles (97–129 kilometers) per hour.