Scientists have discovered something mind-bending about lightning – sometimes its flashes are invisible, just sudden pulses of unexpectedly powerful radiation. Termed "dark lightning", it is now clear to a growing number of lightning researchers and astronomers that along with bright, visible thunderbolts, thunderstorms sometimes unleash sprays of X-rays and intense bursts of gamma rays (called terrestrial gamma-ray flashes, or TGF), a form of radiation normally associated with such cosmic spectacles as collapsing stars. The radiation in these invisible blasts can carry a million times as much energy as the radiation in visible lightning, but that energy dissipates quickly in all directions rather than remaining in a stiletto-like lightning bolt. A person could be struck by dark lightning and they would never even know it.