In 2021, researchers at the University of Zurich issued an unusual invitation: bury your underwear — for science.
What followed was one of the most charming citizen science projects in recent memory. More than 1,000 volunteers across Switzerland took up the challenge, burying over 2,000 pairs of cotton underwear and 12,000 tea bags at roughly 1,000 locations across the country. After two months in the ground, participants dug them up, photographed the remains, and sent the material to the laboratory along with soil samples for further analysis.
The resulting study, recently published in the journal Plants, People, Planet, produced something remarkable: a nationwide map of soil biological activity in Switzerland — one of the most detailed ever created for any country.
The science behind the underwear is elegantly simple. Cotton underwear is made primarily of natural fibres. When buried in healthy, biologically active soil, microbes, fungi, bacteria, and earthworms break it down. The faster it decomposes, the more biological life is present underground — making decomposed cotton a surprisingly useful proxy for soil health.
"Our results show that how soil is managed can significantly affect both soil life and soil quality," said co-study leader Marcel van der Heijden, a professor of agroecology at the University of Zurich. "Healthy, biologically active soil is crucial for fertility, nutrient cycling and many other ecosystem services."
The results revealed striking differences across Switzerland's varied landscapes. Private gardens showed the fastest decomposition rates, meaning the richest underground life. Researchers found that these soils also contained the highest amounts of organic matter, creating favourable conditions for earthworms, fungi, bacteria, and other organisms. Agricultural fields and meadows ranked in the middle, while lawns showed the least biological activity of all — a humbling result for anyone proud of their perfectly trimmed grass.
The findings carry practical implications too. Researchers identified specific practices that encourage soil life: keeping soil permanently covered, applying compost, using more diverse crop rotations, and reducing the use of mineral fertilizers and pesticides. Small changes in garden or farm management, it turns out, can dramatically tip the scales in favour of a thriving underground world.
The project — called Proof by Underpants, or Beweisstück Unterhose in German — was designed with two goals: generate rigorous scientific data, and get the public genuinely excited about what lives beneath the ground. By both measures, it exceeded expectations.
The project also builds on a well-established scientific tradition of using simple materials to track complex ecological processes. The "Tea Bag Index," which uses commercial tea bags to measure decomposition rates, has been applied in studies around the world. The Swiss team paired tea bags with cotton underwear to provide two complementary data streams, giving researchers an unusually rich picture of biological activity at each site.
Together, the data give scientists an unprecedented view of how land use shapes the living world beneath our feet — and how small changes in management can tip the scales dramatically in favour of healthier soil.
Scientists estimate that more than half of Earth's total biodiversity lives underground. For most of human history, that world has been invisible to us. Thanks to 1,000 Swiss volunteers and a great deal of buried cotton, it's a little less so.
The researchers now propose formalising an "underwear index" — a simple, standardised way for anyone to measure their local soil health. It's a whimsical idea with a serious purpose: making the hidden world under our feet legible, one pair of underwear at a time.
