"Why is Crater Lake so blue?"

Answer: The water is so blue because there is hardly anything else in it – just water. It’s not pure water, but it’s close. Water molecules with no sediments, algae, pesticides or pollution, will absorb all the colors of the spectrum except the blues. Those wavelengths will bounce back and make the water appear blue. The key is to have relatively pure water and lots of it. There has to be enough molecules to absorb all the other colors. (There are 4.6 trillion gallons of water in the lake, so it works really well.)