It's common knowledge that the Earth's magnetic poles have reversed numerous times over the past eons. It's these reversals that help us date rock layers and give us proof of sea-floor spreading. Well, what's happened regularly in the past is bound to happen again. But human's won't need to worry for a long long time. A good presentation as are all Nova episodes. Evidently, the Earth's magnetic field has been weakening over the past centuries, as recorded in pottery and rock layers. The scientists go into great detail and use computer modeling to explain what is happening. I really like the soundtrack in this, which is something which usually doesn't catch my attention.
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