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An EMP is caused by the detonation of a high-altitude nuclear bomb which sends a massive surge that fries anything electrical. It destroys anything with a microchip, the entire electric grid, and all vehicles built after the mid-1980s.
McCaul opened the hearing entitled, "Electromagnetic Pulse (EMP): Threat to Critical Infrastructure," saying an EMP would be far more catastrophic than even ground-level nuclear bombs. (watch video below)