In this case, the researchers from Scripps Institution of Oceanography got access to a network of 5,300 geophones (instruments that pick up vibrations in the earth) located in Long Beach, California. The geophones were originally installed as part of an oil and gas survey in the area, but the scientists figured out that in addition to being able to map out likely locations of resources, the network was also able to pick up on the vibrations caused by human transportation.