Here's a rather unusual light source for you. Now I freely admit I may have got the name a little wrong and the explanation certainly went a wee bitty over my head, but I believe what you are looking at is Cherenkovs radiation. What I know for definite is that you are looking at the glowing outline of a cube submerged in a deep tank of water and the glow is due to radioactive decay. Anyway it sure was pretty and I doubt very many folk have seen it let alone been allowed to photograph it. Feel free to provide a better explanation if you can.