Ancient rocks hold proof of Earth's magnetic field. Here's why that's puzzling

The article says "That strength is measured to have been at least 15 microtesla (mT), which is comparable to Earth's field strength of 30 mT today." mT is incorrect; the internationally recognised prefix 'm' stands for for milli (one thousandth), the correct way of expressing a microtesla is μT (one millionth of a Tesla).
It could have been worse, some would have written μT as 'MT' (megatesla).

The unit is "tesla" by the way, the international SI measurement system spells it newton et cetera to not confuse with historical persons (Tesla, Newton).

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