Volcanoes and lightning storms are awe-inspiring, fear-inducing feats of nature. Put them together and you have a truly terrifying event. Volcanic lightning is a mysterious phenomenon that generally occurs at the early stages of a volcanic eruption, and it's taken scientists many years to determine its exact cause. It happens in two places: close to the ground in dense ash clouds, and high up near the stratosphere in the plume of volcanic smoke. For volcanic lightning near the ground, research suggests the cause is the rubbing together of individual ash particles, which builds up enough static electricity to generate a lightning bolt. Sky-high volcanic lightning has a more surprising cause: ice. Scientists think that as the plume of ash and water vapor rises from the volcano, ice begins to form in its highest layers. From there, lightning forms the same way it does in a thundercloud: ice crystals colliding build up enough of an electric charge to trigger a lightning strike. Want to see volcanic lightning in action? Watch the videos below.