According to a news item published yesterday on the website of the journal Nature, evidence from tree rings suggests that a mysterious massive burst of radiation probably struck the Earth about the year 774 or 775, just over 1200 years ago.
This is inferred from finding a jump in carbon-14 levels in one ring (corresponding to the northern hemisphere growing season in 775) that's about 20 times the normal year-to-year variation. The most likely explanation is a flood of gamma rays (high-energy photons) from space in the first part of that year or the latter part of the previous one.
Cosmic rays normally account for the steady production of carbon-14 in our atmosphere. Carbon-14 is mildly radioactive, so as time goes the carbon-14 atoms in a dead plant or animal decay away, so by measuring the amount of remaining 14C it's possible to estimate the age of plant and animal up to about 60,000 years old. A minor consequence of this spike is that plants and animals from that time and a little after would test younger than they really are.
The big mystery is where those gamma rays would have originated. A supernova would account for it, but it would almost certainly have left remnants visible to radioastronomers today, and there's no sign of that. A solar flare and coronal mass ejection is another possibility, but it's not clear that this explanation is viable either.
Thanks to my friend Dave Locke for pointing this out.