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Photo by Avinash Kumar / Unsplash

The first time that Marcus Maeder stuck a noise sensor into the ground, it was on a whim. A sound artist and acoustic ecologist, he was sitting in a mountain meadow and pushed a special microphone he'd built into the soil. "I was just curious," says Maeder, who is working on a dissertation on the sounds of biodiversity at the Swiss Federal Institute of Technology in Zürich, Switzerland.

He certainly wasn't prepared for the clamour of sounds that flooded his headset. "They were very strange," he says. "There was thrumming and chirring and scraping. You need a whole new vocabulary to describe it."

Maeder was eavesdropping, he realised, on creatures that live in the soil.

Ecologists have long known that the ground beneath our feet is home to more life, and more diverse life, than almost any other place on Earth. To a layperson, soil seems little more than a compact layer of dirt. But in fact, the ground is a labyrinthine landscape of tunnels, cavities, roots and decaying litter.

In just a cup of dirt, researchers have counted up to 100 million life forms, from more than 5,000 taxa. Underground denizens range from microscopic bacteria and fungi, pencil-dot-sized springtails and mites, to centipedes, slugs and earthworms that can reach several metres in length. They are joined by moles, mice and rabbits that live at least some of their lives in underground tunnels and dens.

Read the rest at BBC