While many people believe that nothing moves faster than light, "what's actually the case is nothing can go faster than light can go in a vacuum," Wakely explained. But light moves slower in air and water than it does in a vacuum.
"So it's possible that you get a particle which goes through the atmosphere faster than light does. And when that happens, you get the equivalent of a sonic boom, but with electromagnetic radiation. And that light boom is called Cerenkov radiation."