For Nautilus, Charles Digges recounts a massive undertaking begun in 1970 by the Soviet Union: As part of its science program, they decided to dig into Earth. Known as the Kola Superdeep Borehole, at 40,230 feet, it was—and is—the deepest human-made hole on the planet. While the hole didn’t go as deep as they planned, the project gave geologists a new understanding of the subterranean world and the earth’s deep continental crust—a place more alive and dynamic than they ever thought possible.

Other discoveries proved just as momentous. Nearly 22,000 feet beneath the surface, drill cores contained microscopic fossils from two dozen species of plankton that had drifted through an ancient sea more than 2 billion years ago. This showed that even after billions of years of burial deep within the continental crust, unmistakable evidence of some of Earth’s earliest life had survived. Between 16,000 and 23,000 feet, the borehole encountered saline water circulating through fractures in crystalline rocks that geologists had long considered essentially dry. Rather than groundwater that had somehow seeped miles underground, the fluids appear to have originated within the crust itself. As ancient minerals were transformed by heat and pressure, they released chemically bound water, while reactions between water and iron-rich rocks generated hydrogen gas, showing that the supposedly inert continental crust was still chemically active billions of years after it had formed.

“Sound is a tangible way of connecting to aspects of nature that are hidden from humans. The deep ocean, the dark night, and now the dense earth beneath our feet,” writes Ella Browning. And you can hear underground for yourself in the fascinating recordings she includes in her essay on earthworm “munching.” In this emerging field of soil ecoacoustics, researchers—including Browning—are finding links between ecosystem recovery and the character of underground soundscapes. Healthy soil literally sounds different. This piece will change how you view the ground beneath your feet.

A core principle of ecoacoustics is that complexly noisy natural places are healthier natural places. This means that it’s not just the volume of noise, but the diversity of sound that indicates habitat health. The dawn chorus in an intact Bornean tropical forest is magical in its complexity, especially if you’re lucky to experience it mixed in with the whoops of a troop of gibbons swinging through overhead, as I was during a visit in 2019.

In contrast, cities are generally not ecologically healthy and, while they are incredibly noisy places, they are filled with the harsh, more monotonous sounds of car engines and other machines, and often little biotic sound. Soundscapes dominated by anthropogenic sounds have been linked to poor mental health in people, whereas the opposite has been found for natural sounds like birdsong or forest soundscapes.