Coronal holes are a type of hole in the Sun's corona that is continually shifting and reshaping due to the corona's non-uniformity. The Sun has magnetic fields that arch away from locations in the corona that are very thin owing to low quantities of energy and gas, causing coronal holes to form if they do not fall back. The boundaries between these regions of different magnetic properties can move across the surface of the Sun at speeds up to 100 miles per hour. Coronal holes are thought to be the source of many solar flares and coronal mass ejections (CMEs), which are large eruptions of plasma from within the Sun's atmosphere. The presence of a coronal hole does not necessarily lead to an eruption, but it does appear to be the most important factor determining whether or not one occurs. Solar physicists use models based on observations of the Sun to try and predict when eruptions will happen, with the aim of giving us enough time to observe them directly from Earth. Solar physicists used to think that all coronal holes had the same effect on the Sun, but new research shows that this isn't true at all. Some coronal holes cause more activity to occur than others, with some able to shut off our planet's orbit for several weeks at a time while others haven't any effect at all. This means that we need to understand what causes different types of coronal hole to form before we can say for sure how dangerous they will be to Earth.