In nuclear reactors, some particles are forced up to very high speeds, often within a fraction of the speed of light. If they are passing through an insulating medium that slows light down, they can actually travel faster than the light around them.
When this happens, they cause a blue glow, known as “Cherenkov radiation”, which is (sort of) comparable to a sonic boom but with light. This is why nuclear reactors glow in the dark.
Incidentally, the slowest light has ever been recorded travelling was 17 meters per second – about 38 miles an hour – through rubidium cooled to almost absolute zero, when it forms a strange state of matter called a Bose-Einstein condensate.
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