Low-frequency sound waves that cannot be sensed by the human ear. Infrasound was first detected on barometers around the world following the eruption of the volcano Krakatoa in 1883. We now know subaudible sounds are generated by a wide variety of natural phenomena, such as large storms and meteors, and some human-produced sources, such as large chemical or nuclear explosions. Infrasound is particularly useful for basic research, for example, in the study of atmosphere phenomena; as it decays slowly across thousands of kilometers. How infrasound is generated, how it propagates through our turbulent atmosphere, and how it can be recorded amid noise due to air circulation are the subjects of ongoing research.