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Infrasound is low-frequency atmospheric sound generally below the threshold of human hearing. Routinely produced by large-scale unsteady fluid flow, infrasounds can propagate for thousands of kilometers and can provide early warning of natural or man-made hazards. However, these signals are imbedded in a rich ambient sound field and their detection often relies on the processing of four or more microphones arranged as an array. This presentation discusses infrasound from extreme events, which produce very deep sounds. At the lower frequencies (mHz) radiated by the 2004 Sumatra and 2011 Tohoku tsunamigenic earthquakes, the 2013 Chelyabinsk meteor, and the 2014 Kelut volcanic eruption, the whole Earth vibrates. Large volcanic eruptions produce a long-ranging, recognizable acoustic signature that can be used for early warning of eruptions that pose an ash hazard to aircraft. At higher infrasonic frequencies, the ocean also radiates persistently in the Hz range, and can provide supplementary observa
