One of the dangers of volcanoes is that many of them don’t erupt often enough to remind people of the hazards of living nearby. Take Popocatépetl, for example. This active volcano (nicknamed “Popo”) near Mexico City last had a big, explosive eruption about 1,100 years ago. It’s had scores of less violent eruptions since then, and it’s been acting up lately, but that hasn’t been enough to discourage people from living on its slopes. Some even live on top of lava and debris from past big eruptions.

“In many areas, including around Popo, people won’t necessarily believe you when you tell them something is happening,” says Martin Mangler, a geology graduate student at Imperial College London who is studying the