Scientists Warn: Mt Rainier Most Dangerous Volcano

Lahars are unusual because they do not need an eruption to happen. According to the United States Geological Survey (USGS) and the Cascades Volcano Observatory (CVO), Rainier's 25 large glaciers hold more than five times as much snow and ice as all other Cascade volcanoes combined. Heavy rain, a sudden warming, or even a dam failure can release this material and trigger a "no-notice" lahar that may grow hundreds of feet tall and travel more than 50 miles. Volcanologist Lizeth Caballero García explained that lahars are complex: they can grow, weaken, and change as they move down the mountain.

The danger is not just theoretical. In November 1985, a lahar from Colombia's Nevado del Ruiz volcano buried the town of Armero and killed more than 23,000 people within minutes. To prevent a similar disaster, the CVO has placed seismic sensors across the Cascade Range to detect lahars and earthquakes and alert emergency teams. Two years ago, more than 45,000 students and staff from over 20 schools west of Rainier joined one of the world's largest lahar evacuation drills. Officials say running uphill on foot is still the best way to escape.