Nevado del Ruiz, Columbia. On 11/13/1985, a relatively small explosion unleashed floods that swept away 1500 people on one side of the mountain. On the other side was the town of Armero, the site of the worst destruction. 25 mph lahars obliterated the town and blanketed it in gray muck. When the landslides subsided, 23,000 people had died and damage was estimated at 1 billion dollars -- one-fifth of Columbia's GNP at the time.