The lives that slither and scurry beneath the soil’s surface are vital for ecological health: now we can listen in to them

by Ella Browning  BIO

Animation by Dulcie Yamanaka

is an interdisciplinary ecologist with a passion for ecological sound, based in the Leverhulme Centre for Nature Recovery at the University of Oxford as a postdoctoral researcher. Using passive acoustic sensors to monitor animals such as bats, crickets and recently earthworms, she aims to understand how biodiversity is faring in response to human-driven environmental change and identify effective nature recovery approaches. Ultimately, she hopes to improve outcomes for nature and people.

Edited byRichard Fisher

Earthworms do not have a discernible mouth, a means to make sound, or ears, and most spend their lives burrowing below ground, eating soil and decaying organic matter. However, their behaviours produce sounds, which can reveal a surprising amount about the subterranean world in which they live. I’ve recently found myself leaning over a box of soil in the basement of my university department, headphones clutched to my ears, revelling in the soft, Velcro-like scrapes of earthworms eating. It sounds like this:

Spectrogram of an earthworm ‘scrape’ recorded in Oxfordshire grassland

I have always loved earthworms. As a child on my parents’ allotment, I delighted in finding them in the soil as potatoes were dug in, fascinated by their soft, alien form wiggling in my hand. Now, I amuse my friends by ducking down in crowds at music festivals to rescue distressed earthworms from being squashed by the mob of muddy boots.

Until recently, my scientific research focused on the rapid patter and chop of bat echolocation calls, monitoring how their populations were responding to largely human-driven environmental change. I didn’t consider that the same ecoacoustic approaches could be applied below ground, to understand how earthworms are affected by us surface-dwellers. Fortunately, others had. I finally had a way to connect my scientific research speciality with my ardour for earthworms in the soil.

Worm munch might seem like a curiosity, but it represents a much bigger change in how we understand nature. More than just the behaviour of single creatures, such sounds can also reveal the health of whole ecosystems – and the underground is emerging as a new acoustic frontier.

Sound itself is a wave, a travelling vibration. Vibrancy is a word often used to describe colour and life. A vibrant ecosystem, therefore, is one that hums with the sounds of life.

While reading this essay, you can listen to a 15-minute recording of various sounds of the underground, collected in Oxfordshire in 2024:

Pasture gradient sounds medley recorded in an Oxfordshire field

Monitoring nature’s sounds has a long history. The ancient Greeks wrote poems and plays about birdsong and, in medieval England, the sounds of local birds gave many places their names. Today, an expert ornithologist will stand in a forest and conduct a bird survey using nothing but their eyes and ears.

However, when bioacoustics researchers in the 20th century began to place microphones or hydrophones and recorders in the field for days, weeks or months – known as ‘passive acoustic monitoring’ – they discovered whole new natural soundscapes. This technique has been especially powerful in realms where humans cannot stay for long, or for species that emit infrasonic or ultrasonic sounds beyond human hearing range (20-20,000 Hz).

Underwater habitats, for example, are difficult for people to be immersed in for extended periods without specialist equipment and training (for obvious reasons). Thus, collecting data on animals like whales and dolphins, which can traverse vast oceanic distances, was historically done with sightings from land or boats. That changed with passive acoustic sensors.

Bats use the returning echoes of their ultrasonic calls to build a sonic map of their surroundings

In the 1950s during the Cold War, the US Navy developed its Sound Surveillance System (SOSUS), an underwater network of hydrophones with the purpose of tracking Soviet submarines in the Atlantic and Pacific oceans, originally code-named ‘Project Jezebel’. Arrays of hydrophones were deployed on the seafloor, connected via cables to the shore where the acoustic data could be monitored. While an original premise of this system was upheld – that enemy submarines would be easily tracked by their low-frequency sound signatures – the assumption that there would be little other noise in the ocean was not true. Sailors operating the SOSUS detected many other unknown sounds, including one they attributed to the ‘Jezebel Monster’. It took some years and the detective work of marine bioacousticians for this to be identified as the 20 Hz call of fin whales. Many other mysterious sounds began to be discovered, including the ‘boing sound’ and the ‘lightsabre sound’, both of which were also eventually attributed to minke whales. Since many cetaceans use sound for........

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