What Causes Halos Around the Sun?

There seems to be more and more sightings of this strange phenomenon. The question is, what is causing it, and is it natural? A few years ago, a ring around the sun actually sparked fear among many people in Mexico. People have even called 911 to report this strange occurrence. Sometimes at night, a similar halo can be seen around the moon on a cloudless night. Some scientists have explained that this phenomena is caused by ice crystals in the upper atmosphere and it is more common than we think. The reason it so commonly goes unnoticed, they say, is because we simply don't look up enough. Could that really be the reason, or could it be related to geoengenieering?