Whereas ultrasonic sounds are frequencies above the human bandwidth of hearing, infrasonic (or “subsonic”) sounds are those frequencies below the human bandwidth of hearing, or 20 hertz. Infrasounds as low in frequency as 0.001 hertz are produced by a variety of natural sources like earthquakes and volcanoes, as well as man-made structures and nuclear explosions. Elephants are known to make sounds as low as 12 hertz, and can communicate in this way over large distances.