Infrasound refers to sound waves that are below the frequency of 20 hertz, which is the lower limit of human hearing. These low-frequency sounds can be produced by natural events, such as earthquakes, volcanic eruptions, and ocean waves, as well as by man-made sources like machinery and explosions. Infrasound can travel long distances and penetrate various materials, making it useful for monitoring geological activity and detecting nuclear tests. While humans cannot hear infrasound, it can still affect the body, potentially causing feelings of unease or anxiety in some individuals when exposed to high levels.