His activity Nevado del Ruiz Volcanoin the central Colombiahas intensified in recent days and local authorities have warned that it could explode.

The Ministry of Mines, in an “emergency announcement” issued overnight Thursday into Friday, said it had raised the alert level to “orange,” meaning the volcano is likely to erupt within the next few days or weeks.

Nevado del Ruiz is etched in Colombian memory for the nightmarish tragedy of Armero, the town that was buried under tons of mud and volcanic material in 1985, killing nearly 25,000 people. The martyrdom of 13-year-old Omaira Sanchez, who was trapped in a hole covered with water and was slowly dying for three days on live broadcast, shocked the whole world at the time.

“Since March 24, seismic activity has increased significantly” at the stratovolcano, whose altitude exceeds 5,300 meters, according to the ministry. On March 28, 6,500 earthquakes were recorded and the following day more than 11,000. “By the time this announcement was written, more than 9,600 earthquakes had been recorded today (Friday), the ministry added.

Authorities advised residents to remain calm but to be regularly updated by the Colombian Geological Survey. The country’s president, Gustavo Pedro, called on mayors to “prepare the protocols for this level of alert”.

Well-known examples of stratovolcano eruptions in the world are those of Krakatoa in Indonesia in 1883 and Mount Vesuvius which buried Pompeii and Herculaneum in 79 AD.

According to the US Smithsonian Institution’s World Volcanology Program, the death toll from the 1985 Nevado del Ruiz activation was the fourth highest ever recorded worldwide from a single volcanic eruption.