Nine times in the past 4,000,000 years or so, the earth's magnetic field has completely reversed. The North Pole became the South Pole, the South Pole the North. During these reversals, scientists theorize, the intensity of the earth's magnetic field actually decreased to zero, then built back up with opposite polarity. During the temporary absence of a strong magnetic field, the earth was left without its protective shield; as a consequence, cosmic-ray particles that were normally deflected by the magnetic field could shower through the atmosphere. The result may have been the destruction, mutation or even creation of species....
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