European mains electricity oscillates at 50 Hz… give or take a few thousandths: its frequency keeps varying with the balance between production and consumption. A microphone near a plugged-in appliance picks up that hum. Its curve, second by second, is a fingerprint that can be found in the grid's records: that is how a recording can be dated. Available records: 14 to 17 October 2026 (one measurement per second). Everything happens in your browser.
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In real life: ENF (Electric Network Frequency) has been used by forensic labs since the 2000s (United Kingdom, Netherlands…) to date recordings and spot edits; frequency records are published by grid operators (RTE in France, data.rte-france.com).