“Let me start off with a riddle,” says NASA scientist Allan J. Zuckerwar. In his office in Hampton, Va., he rattles off items as dissimilar as rhinoceroses, supersonic aircraft, and hurricanes. “Now, what do they have in common?” The answer, Zuckerwar explains, is that each one generates silent infrasound—long sound waves at a frequency below 20 hertz. People can’t hear anything below that frequency, probably for good reason. Otherwise, they’d be bombarded by the constant din of wind, the intermittent groaning of Earth, and the occasional distant explosion. But scientists are eavesdropping on volcanoes, avalanches, earthquakes, and meteorites to discern these phenomena’s infrasound signatures and see what new information infrasound might reveal.
Just as seismic waves travel through Earth, infrasonic waves travel through the air. And the lower the frequency of the waves, the farther they can travel without losing strength. Scientists first detected infrasound in 1883, when the eruption of the Krakatoa volcano in Indonesia sent inaudible sound waves careening around the world, affecting barometric readings.