What causes mudslides? That simple question is suddenly a very poignant one, given the deadly landslide that devastated the small community of Oso in Washington state on March 22 -- and the answer might not be what you'd expect.
Mudslides often have many contributing factors. Just check out this recent New York Times video (above). It points out that mudslides, or debris flows, can be caused simply by gravity -- though they are often triggered by earthquakes or fires.
Why fires? When trees burn, their root systems may also be destroyed. And roots tend to keep soil and mud in place even on steep hillsides.
Recent heavy rainfall and soil saturation likely caused the mudslide in Washington, Rex Baum, a research geologist with the U.S. Geological Survey, told National Geographic.
Seismic readings have shown that there were actually two major slides about four minutes apart during Saturday's disaster, and smaller slides continued for a few days.
What remains after a mudslide is just as dangerous. The debris field in Washington is about one square mile and 30 to 40 feet deep in places, with treacherous terrain of quicksand-like muck, mud and ice.