When Charlemagne was ruling France, something struck Earth. We know it struck because a radioactive carbon-14 spike was recorded in tree rings. This was reported in Nature today, but the researchers cannot explain it. The two likeliest explanations are supernovas or solar flares, but scientists do not have any evidence to support one or the other (no supernovas were observed and solar flares are probably too weak). But whatever it was, our planet was surely bombarded by bursts of high radiation.