Once-crowded city streets got empty. Highway traffic slowed to a minimum. And fewer and fewer people can be found milling about outside.
Global containment measures to combat the spread of the coronavirus have seemingly made the world much quieter. Scientists noticed it, too.
Around the world, seismologists observed a lot less ambient seismic noise -- meaning, the vibrations generated by cars, trains, buses and people going about their daily lives. And in the absence of that noise, Earth's upper crust moved just a little less.
Thomas Lecocq, a geologist and seismologist at the Royal Observatory in Belgium, first pointed out this phenomenon in Brussels.
Brussels saw about a 30% to 50% reduction in ambient seismic noise in the mid-March, around the time the country started implementing school and business closures and other social distancing measures, according to Lecocq. That noise level is on par with what seismologists would see on Christmas Day, he said.
The reduction in noise has had a particularly interesting effect in Brussels: Lecocq and other seismologists were able to detect smaller earthquakes and other seismic events that certain seismic stations wouldn't have registered.
Seismic stations were typically set up outside urban areas, because the reduced human noise makes it easier to pick up on subtle vibrations in the ground. The one in Brussels, however, was built more than a century ago and the city has since expanded around it.
The daily hum of city life means that the station in Brussels wouldn't typically pick up on smaller seismic events. Seismologists would instead rely on a separate borehole station, which uses a pipe deep in the ground to monitor seismic activity.
"But for the moment, because of the city's quietness, it's almost as good as the one on the bottom," Lecocq said.
Seismologists in other cities saw similar effects in their own cities.
Paula Koelemeijer posted a graph on Twitter showing how noise in West London has been affected, with drops in the period after schools and social venues in the United Kingdom closed and again after a government lockdown was announced.
Celeste Labedz, a PhD student at the California Institute of Technology, posted a graph showing an especially stark drop in Los Angeles.