Worms, grubs, and roots rummaging beneath our feet produce a cacophony of sounds that we are only now beginning to listen to in order to learn more about life underground.
Marcus Maeder planted a noise sensor in the ground on a whim the first time. He sat in a mountain meadow, a sound artist and acoustic ecologist, and pushed a special microphone he’d built into the soil. “I was just curious,” says Maeder, who is working on a dissertation on biodiversity sounds at Switzerland’s Swiss Federal Institute of Technology in Zürich.
He wasn’t expecting the cacophony of sounds that flooded his headset. “They were very odd,” he says. “There was a lot of thrumming, chirring, and scraping. To describe it, you’ll need a whole new vocabulary.”
Maeder realised he was listening in on creatures that live in the soil.
Ecologists have long known that the ground beneath our feet supports more and more diverse life than almost anywhere else on the planet. To the untrained eye, soil appears to be nothing more than a compact layer of dirt. In reality, the ground is a maze of tunnels, cavities, roots, and decaying litter.
Researchers have discovered up to 100 million life forms from over 5,000 taxa in just a cup of dirt. Underground life includes everything from microscopic bacteria and fungi to pencil-dot-sized springtails and mites to centipedes, slugs, and earthworms that can grow to be several metres long. They are joined by moles, mice, and rabbits, all of which spend at least part of their lives underground in tunnels and dens.
“It’s mind-boggling,” says Uffe Nielsen, a soil biologist at Western Sydney University in Australia. It’s also an important one, because these subterranean communities provide much of the foundation for life on our planet, from the food we eat to the air we breathe.
Today, in a relatively new field known as soil bioacoustics – other terms include biotremology and soil ecoacoustics – a growing number of biologists are recording underground noises in order to gain insight into this complex and cryptic world. They discovered that a simple metal nail pushed into the ground can function as an upside-down antenna if equipped with the right sensors. And the more researchers who pay attention, the better. The deeper we go, the clearer it becomes how alive the ground beneath us is.
Eavesdropping on this cacophony of underground sounds promises to reveal not only what life forms live beneath our feet, but also how they live – how they eat or hunt, how they slither past each other unnoticed, or drum, tap, and sing to get each other’s attention. Nielsen describes underground life as “a black box.” “We realise how little we know as we open it.”
Understanding this underground life is critical because soil ecology is so important. “Soil helps to transform the nutrient elements like carbon, nitrogen, phosphorus, and potassium that feed plants – for food, for forests, or to fill the air with oxygen so we can all breathe,” says Steven Banwart, a soil, agriculture, and water researcher at the University of Leeds in the United Kingdom, who co-authored an overview of soil functions in the Annual Review of Earth and Planetary Sciences. Worms, grubs, fungi, bacteria, and other decomposers are present at all stages.
And each soil organism has its own soundtrack. As they break down the fibres of their meal, root-munching larvae emit short clicks. As they crawl through tunnels, worms rustle. Plant roots, too, push past soil grains, as Swiss researchers reported in 2018. However, the roots move more slowly and steadily than the worms. Soil acoustics has the potential to shed light on previously unanswered questions by distinguishing these sounds. For example, when do plant roots grow? Late at night? During the daytime?
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We humans may be the last to discover this hidden soundtrack. Birds are frequently observed hopping across lawns with their heads cocked. Researchers believe they do this to listen for worms or larvae beneath the surface. They frequently peck at the soil at precisely the right moment to pull up their unsuspecting prey.
The North American wood turtle, for its part, takes advantage of worms’ attention to vibrations from raindrops. The turtle stomps its feet on the ground to imitate that patter, causing the worms to surface and provide a juicy snack.
Subterranean vibrations can also play an important role in what appear to be intended signals. Rats, Mole Living in underground burrows, they are thought to communicate with other mole rats nearby by banging their heads or feet against the tunnel walls. When buried during nest cave-ins, leafcutter ants have been observed making noises. Other worker ants rush to the location and begin digging to save their nestmate.
Some of these underground sounds can be heard by humans, but many are too high or low in frequency (as well as in volume). To capture these, researchers use tools such as piezoelectric sensors, which function similarly to contact microphones that can be clipped onto a guitar. Attached to a nail that has been pushed into the ground, these sensors detect vibrations, which researchers then convert into electronic signals and amplify until humans can hear them.
Carolyn-Monika Görres, a landscape ecologist at Geisenheim University in Germany, was surprised to learn how much underground noise can reveal. Görres studies white grubs, which are root-feeding beetle larvae. She is particularly interested in the gases they emit, such as methane. Because of their sheer numbers, biologists believe that these small insects of various species contribute significant amounts of climate emissions. (Termites, for example, are estimated to account for approximately 1.5% of global methane emissions.) In comparison, coal mining accounts for 5-6% of the total.)
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