The eruption of Nevado del Ruiz in Colombia on 13 November 1985 was accompanied by the formation of four lahars (mud flows) triggered by the melting of glacial ice near the summit of the volcano. The lahars began as flows of water, sand and gravel, but they incorporated clay by eroding the soil along the steep valleys through which they passed. The largest flow was a cohesive debris flow, more than 45 m deep and moving at ∼12 ms−1 when it debouched from the canyon of the Rio Lagunilla 2.5 km from Armero, where it killed 25,000 people. The continuing volcanic activity and abundant remaining glacial ice create an extremely high risk of future destructive flows.