The Crab Nebula has shocked astronomers by emitting an unprecedented blast of gamma rays, the highest-energy light in the Universe. The cause of the 12 April gamma-ray flare, described at the Third Fermi Symposium in Rome, is a total mystery. It seems to have come from a small area of the famous nebula, which is the wreckage from an exploded star. The object has long been considered a steady source of light, but the Fermi telescope hints at greater activity. The gamma-ray emission lasted for some six days, hitting levels 30 times higher than normal and varying at times from...