Javanese for rapid mudﬂow associated with volcanic activity, a lahar is created when volcanic ash is converted into a mobile paste by water from torrential rain, snowmelt, or breaching of the walls of a crater lake. This ﬂuid landslide may spread many miles from the source volcano when conﬁned along preexisting valleys: the ﬁve-thousand-year-old Osceola Lahar of Puget Sound in Washington is up to ﬁve hundred feet thick and extends over two hundred square miles. When the Electron Lahar that swept from Mount Rainier about ﬁve hundred years ago was excavated in 1993, remnants of an old-growth forest were exposed. Associated with lahars are nuées ardentes, French for glowing clouds—highly mobile, turbulent, and sometimes incandescent clouds erupted from a volcano and containing large amounts of ash and other pyroclastics. Such a mixture, once it settles, is quite ﬂuid, and it can move rapidly down even a gentle slope. When the mixture comes to rest, the hot glass fragments meld together, forming a welded tuff. It was an eruption of this kind from Mount Vesuvius that buried the city of Pompeii in A.D. 79. In 1985, more than twenty-three thousand people were killed by a lahar that swept through Armero, Colombia. Lahars are, in fact, often catastrophic and the cause of most volcanic fatalities, which is why the United States Geological Survey recently instituted a pilot project in Washington called the Mount Rainier Volcano Lahar Warning System.