Apparently electrical outlets give off an inaudible hum, which isn’t all that interesting in and of itself. Except that that hum changes frequency in minute ways constantly based on the supply and demand of electricity. Scientists in the UK have discovered that the hum is unique, which means it can be used to timestamp recordings. The gist:
Recordings made close to electrical power sources pick up a hum. Comparing the unique pattern of the frequencies on an audio recording with a database that has been logging these changes for 24 hours a day, 365 days a year provides a digital watermark: a date and time stamp on the recording. Philip Harrison, from JP French Associates, another forensic audio laboratory that has been logging the hum for several years, says: “Even if [the hum] is picked up at a very low level that you cannot hear, we can extract this information.”
Science is pretty crazy sometimes.