Infrasound is one way scientists at UAF Geophysical Institute and the Alaska Volcano Observatory monitor and study volcanoes. When volcanoes have explosive eruptions, they emit a sound wave that can be recorded on special microphones. This sound wave is too low for humans to hear (infrasound) but can be detected on instruments far away from the source. The map and figure below show an explosion at Cleveland Volcano and the seismic signal (measurement of the ground shaking) compared to the infrasound signal (red arrow) at stations over 100 km away from the volcano (red arrow on map).