Beautiful - but deadly - blue glow (i.e., Cherenkov radiation) of an operating or "critical" nuclear reactor seen under many feet of radiation shielding and thus life protecting water. The light is given off because highly charged particles of matter (e.g., electrons) around the reactor are traveling faster than the speed of light in water (the speed of light in water is 75% of the speed of light in a vacuum). While the matter of the charged particle can exceed the speed of light in water, the charge must travel at the local speed of light. This causes the charge on the particle to lag behind the matter of the particle with the consequence that photons in the blue to an ultraviolet region are emitted. Think of this as a photonic boom similar to the sonic boom of a jet traveling faster than sound in the air!
Myth buster: Those that work around nuclear power do not glow green - they glow blue! (Just kidding!).