Nevado del Ruiz
The eruption was small — in volcanic terms, that is — producing only about 3% of the ash ejected by Mount St. Helens in 1980. Instead, it was the mudflows that made Colombia's 1985 Nevado del Ruiz explosion the second deadliest in the 20th century and the fourth deadliest in recorded history. The volcano has been blowing its top since the Pleistocene era and has erupted three times in modern history, including in 1595 and 1845. But on Nov. 13, 1985, a relatively small explosion unleashed floods that swept away 1,500 people on one side of the mountain. On the other side was the town of Armero, the site of the worst destruction. 25-m.p.h. lahars (volcanic mudflows) 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 — one-fifth of Colombia's GNP at the time.