Most of us take for granted that what we can hear represents the full acoustic world around us. It doesn't. Beneath the threshold of human perception lies an invisible ocean of sound — and scientists are just beginning to learn how to navigate and exploit it.
Called "infrasound," these ultra-low-frequency vibrations fall below 20 hertz, the lower boundary of normal human hearing. They travel through the atmosphere, oceans and solid ground, carrying remarkable information about natural events and human activity across vast distances. The Earth, it turns out, is far from quiet.
What makes infrasound so extraordinary is how it behaves physically. Low-frequency sound waves have very long wavelengths, which allows them to travel across continents and oceans while losing very little energy. Unlike ordinary sound, infrasound bends around obstacles and penetrates buildings. A volcanic eruption in Alaska or a meteor strike over Russia can be detected thousands of miles away — not by satellites or radar, but by microphones sensitive enough to pick up the faintest ripple in atmospheric pressure.
Nature produces these signals constantly. Ocean waves colliding far offshore generate persistent atmospheric vibrations that can circle the globe. Earthquakes, avalanches, tornadoes and severe storms all carry infrasonic signatures. So do human activities: rocket launches, mining blasts, industrial machinery and nuclear detonations all leave a distinctive low-frequency fingerprint.
Animals figured this out long before we did. Elephants use infrasonic calls to coordinate herd movements across miles of savanna. Whales communicate across entire ocean basins using low-frequency pulses that travel farther than any human voice could dream of reaching.
Scientists are now using infrasound as a powerful monitoring tool. The U.S. Geological Survey routinely deploys infrasonic sensors near volcanoes, particularly in remote locations such as Alaska, where visual observation is impossible. Explosive eruptions generate distinctive pressure waves that help researchers estimate eruption intensity and identify ash-producing events — critical information for aviation safety and emergency management.
Meteor detection is another growing application. When space rocks enter the atmosphere, they produce shock waves detectable thousands of miles away. Researchers at NASA and elsewhere use these signals to estimate a meteor's energy, speed and trajectory. The 2013 Chelyabinsk meteor over Russia, which injured more than 1,500 people, produced one of the largest infrasonic events ever recorded, picked up by monitoring stations worldwwide.
Scientists are also exploring whether infrasound could one day improve tornado warnings. Funnel formation appears to generate characteristic low-frequency signals before a tornado touches down, raising the possibility that infrasonic detection systems could supplement radar and give communities precious additional seconds of warning.
Perhaps the most surprising use of infrasound is geopolitical. The Comprehensive Nuclear-Test-Ban Treaty Organization operates a worldwide network of infrasonic monitoring stations as part of its effort to detect secret nuclear weapons tests. These stations, equipped with extraordinarily sensitive instruments, can pick up atmospheric nuclear explosions from virtually anywhere on Earth.
As a fringe benefit, this arms-control infrastructure has become one of the most valuable scientific observatories ever built — continuously capturing data from volcanic eruptions, meteor strikes, severe storms and rocket launches. A surveillance network built to keep nations honest has ended up teaching us how our planet breathes.
Advances in sensor technology and machine learning are making it possible to extract meaningful signals from the constant low-frequency hum of the atmosphere. Artists are joining scientists in exploring what this hidden world sounds like. Brian House, a "sound artist" and professor at Amherst College, has built what he calls "macrophones" — devices modeled on nuclear-test monitoring equipment that capture planetary vibrations and translate them into sounds humans can actually hear.
The emerging science of infrasound is a reminder that human perception captures only a narrow slice of reality. Below our everyday experience of sound, the Earth maintains a constant, hidden symphony — oceans talking to the atmosphere, volcanoes announcing their fury across continents, storms whispering their approach long before they arrive.
Henry I. Miller, a physician and molecular biologist, is the Glenn Swogger Distinguished Scholar at the Science Literacy Project. He was the founding director of the FDA's Office of Biotechnology.