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Tree rings can tell you a lot: The Japanese have discovered that, in the late eighth century, the earth was hit by a mysterious blast of cosmic rays (NOTE: Subscribers can still listen to this show). They discovered this by looking at cedar trees.

An analysis of two ancient trees found a surge in carbon-14, an isotope that arrives from space, which was deposited in cedar tree rings only in 774 and 775 AD. What happened?
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