You may not have been dreaming when you thought you heard a loud sound in the early morning. The phenomenon is called cryoseism and was also reported on Christmas eve around the GTA. Why is this happening and what is cryoseism? Cryoseism occurs when the ground and rock contains a significant amount of water, and as the temperatures rapidly dip, the water freezes expanding by 9%, putting stress on the surrounding rock. The stress releases explosively creating a boom as the ground cracks. The Great Lakes area seems to record many of these events, and the frequency of the ones here in Toronto have been attributed to the ice storm and the very cold temperatures we have been experiencing this winter.