Nevado del Ruiz is a 5,389-metre high volcano in Colombia, South America. The town of Armero is 74 kilometres away. The volcano had erupted before, in 1845. That eruption killed hundreds of people in Armero. But, by 1985, most people had forgotten about the dangers of the volcano and the town's population had grown to almost 30,000. Scientists had known for over a year that the volcano was active, but when they tried to warn people of the danger, nobody believed them.