There are some volcanoes that we know really well. They've erupted recently enough that we had well-developed geological monitoring technology to analyze them, and we've gained an understanding of exactly how they behave when they're gearing up to blow. Most, however, haven't erupted in a long time, so when the ground starts rumbling, we don't know how long it will be before the grand finale. But scientists have discovered that tiny crystals left behind from past eruptions can give us a sense of the volcano's timeline, and that's crucial information for emergency planning. Find out all there is to know about volcanoes below.