The volcanic eruption in Colombia’s Tolima department, Nov. 13, 1985, was not only a tragedy due to the immense death toll — it left 25,000 people dead and is South America’s deadliest ever volcano eruption and the second most deadly eruption of the 20th century — but also because the Colombian government had received a warning in September that the area should be evacuated due to the Nevada del Ruiz’s recent activity.

The United States Geological Survey explains that, “A visiting UN geologist advised the installation of monitoring equipment and the creation of a hazard map and evacuation plans. But Colombian scientists lacked the expertise, government support, and equipment necessary to effectively monitor the volcano and relay information to public authorities.”

The eruption caused massive mudflows to quickly eat up nearby towns, with Armero being the worst hit. Residents did not hear the volcano as there was a storm over the area, too, which also restricted communication once the danger was understood. 

The USGS explains: “Nevado del Ruiz is a stratovolcano, akin to Washington's Mount St. Helens, topped by glaciers, rising 5,389 m (17,784 ft) above sea level. Such volcanoes are especially dangerous, because heat from the eruptions melts the ice to create lahars, or mudflows of volcanic debris. These are not slow, cumbersome mudflows, but fast, deep, and destructive walls of debris and water.