Besides the roar of engines and honking of angry drivers, rush-hour traffic also makes underground “noise.” We can’t hear most of these ground vibrations, but seismic sensors can. With a network of 5300 geophones—devices that convert ground movements into voltage—researchers recorded 1 week’s worth of urban vibrations in a 70-km2 area of Long Beach, California. By analyzing the seismic data, they could measure how fast individual trains were moving between stations, count the number of planes landing and taking off at the airport, and calculate the average speed of vehicles on a 10-lane highway. Without GPS or cameras, seismic systems could allay privacy concerns by tracking urban activity in an anonymous way, researchers reported today at the 168th Meeting of the Acoustical Society of America in Indianapolis. In addition to monitoring vehicular traffic, vibrational data from seismic sensors could help researchers track building use and identify industrial, residential, or office zones within cities.