Every so often, Earth's magnetic field flips on its head, turning the magnetic North Pole into the South Pole and vice versa. It last happened 780,000 years ago, and is predicted to occur again in about 1,500 years... maybe.
The overall frequency is hard to predict - there was one period in Earth's history when the field didn't reverse for 30 million years. Why these flip-flops happen at all is a great riddle, but a new hypothesis on the origins of the magnetic field could shed light on the reason.
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