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The Core
In the movie, the Earth's inner core -- a nickel-iron mass about 1500 miles in diameter -- stops rotating, causing the planet's magnetic field to collapse and microwave radiation from space to blast through the atmosphere. But microwaves aren't affected by magnetism, and the radiation that comes from space is too weak to damage anything here. What's more, if the core did stop rotating for whatever reason, we'd have more to worry about than that. The energy stored in the core would have to go somewhere, and the effect on the planet would be equivalent to five trillion nuclear bombs going off at once.
(source: movies.yahoo)