It's been about 700 000 years since the last time the Earth's magnetic field reversed its polarity. Considering that the average over the last 30 million years has been about four geomagnetic reversals per million years, we're a bit overdue. But if Berkeley physicists Richard Muller and Donald Morris are right, the onset of the next reversal will be more than just an inconvenience to navigators. In the November issue of Geophysical Research Letters, they make an intriguing, if rather speculative, case for the thesis that many, perhaps all, polarity reversals of the dynamo churning in our planet's liquid‐iron outer core are caused by the impact of large extraterrestrial objects falling to Earth.

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