Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures tomitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity,leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is thelongest and most prominent global anthropogenic seismicnoise reduction on record. Although the reduction isstrongestatsurfaceseismometersinpopulatedareas, this seismic quiescence extends for many kilometersradially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signalsfrom subsurface seismic sources that would have been concealed in noisier times and to benchmark sources ofanthropogenic noise. A strong correlation between seismic noise and independent measurements of humanmobility suggests that seismology provides an absolute, real-time estimate of human activities.