For years, seismologists in Italy have puzzled over odd and persistent seismic signals, seemingly caused by subterranean magma movements but far from active volcanoes. Now, a new analysis has unveiled a concrete and much more prosaic trigger: the machinery in huge cement factories. Researchers analyzed the patterns of hundreds of low-frequency rumbles and discovered ties to many of the country's 57 cement plants. For instance, 342 events (yellow circles) occurred within 150 meters of the Ghigiano factory. Moreover, there were far fewer occurrences at night, on weekends, and in August, when the giant plants are shuttered. The vibrations may rattle up to half the country, researchers report online this week in Geophysical Research Letters, so seismologists must take them into account when they analyze other low-level seismic signals—yet another example of earthly noise caused by humans.