Nevado del Ruiz, Colombia

An active volcano at elevation 5,321m. It is notorious for a 1985 event where an eruption led to a mud stream down the flanks of the volcano that would erase the town of Armero (situated a stunning 5,000m lower), leaving 23,000+ death. Especially tragic was that those in the know of it coming, failed to get through to the national and local authorities whom framed it as "scaremongering". Next, as it became absolutely sure that the town would be hit, the town's electricity grid failed, making communication impossible.

https://en.wikipedia.org/wiki/Armero_tragedy