Sound waves in the atmosphere at frequencies below the hearing threshold of humans are termed infrasound. These subaudible waves are generated by a wide variety of natural phenomena, such as volcanic eruptions, and some human-made sources, such as large chemical or nuclear explosions. Infrasound has become an important component of the global effort to monitor nuclear testing activity. A global network of 60 infrasound arrays is currently being constructed for this purpose. Infrasound is a particularly valuable remote-sensing tool since it decays slowly across thousands of kilometers.