Beneath our feet, threading through the top six inches of soil on every continent, runs a network so vast that its total length, laid end to end, would stretch roughly one billion times the distance from Earth to the Sun. This is the hidden infrastructure of arbuscular mycorrhizal (AM) fungi, and for the first time, scientists have mapped it at global scale.
The map, published June 11 in Science and released as an interactive visualization this week, compiles data from 322 published studies encompassing more than 16,000 soil cores across nine biomes, from deserts to tundra to tropical forests. Using random forest models trained on environmental layers and run on Google Earth Engine, the research team, led by Justin Stewart and Corentin Bisot of the Society for the Protection of Underground Networks (SPUN) and the AMOLF Biophysics Institute, produced the first comprehensive estimate of the global density and biomass of AM fungal networks.
The total hyphal length in the top 15 centimeters of soil is 1.10 × 10¹⁷ kilometers, 110 quadrillion km, or approximately 68 quadrillion miles. In a single teaspoon of healthy soil, up to 10 meters of these microscopic fungal threads can be found.
The corresponding biomass is approximately 300 megatons of carbon, with an uncertainty of ±60 megatons. That is four to six times the mass of all living humans. The lead author, Stewart, said he and his colleagues “spent two or three weeks recalculating to make sure we weren’t missing zeros” because the biomass seemed surprisingly modest relative to the enormous length, an indication of just how thin individual hyphae are.
Each year, these networks move an estimated 1 billion metric tons of carbon into soils, equivalent to roughly 4 billion tons of CO₂, or about 11% of human-related annual CO₂ emissions.
The architecture of the network
AM fungi form symbiotic relationships with the roots of roughly 80% of land plants. The fungus provides phosphorus, nitrogen, and water to the plant in exchange for carbon compounds produced through photosynthesis. The “network” is not a single interconnected web but the collective hyphal length of many individual fungal individuals, microscopic filaments, thinner than a human hair, that branch through soil particles.
The team calibrated their biomass model using a robot named “Prince” (alongside lab-mates “Donna Summer” and “Aretha Franklin”) at AMOLF in Amsterdam, which imaged more than 300,000 living AM fungal hyphae grown on transparent media, measuring network length at high resolution.
Where the networks are
Grasslands hold the largest share, approximately 40% of the global AM fungal infrastructure. The researchers identified three biodiversity hotspots for AM fungi: the flooded grasslands of South Sudan, Florida’s Everglades, and the Tibetan plateau.
But a finding that unsettled the team is that cropland soils have roughly 50% lower network densities than their natural counterparts on average. With grasslands being converted to farmland at four times the rate of forests, the loss of this underground infrastructure may be accelerating faster than current estimates capture.
“We still know relatively little about what they do, how effectively they function, and how their roles vary across environments and host plants,” said Katie Field, a co-author and Royal Society Faraday Discovery Fellow at the University of Sheffield, whose new fellowship will investigate these functional questions.
Conservation and climate implications
The findings arrive as awareness of fungal networks’ role in planetary health is growing. A companion 2025 paper from the same research group found that 95% of AM fungal biodiversity hotspots lie outside any protected area.
The team’s interactive map, designed by Moritz Stefaner (Truth & Beauty) and available at a-hidden-infrastructure.spun.earth, reveals not only the distribution of AM fungal density but also the regions most severely undersampled, serving as what the researchers call “treasure maps of where we need to go sample.”
Caveats
Several important limitations apply. The estimate covers only the top 15 centimeters of soil, deeper fungal networks are not included, which may significantly underestimate total fungal presence. The map covers only arbuscular mycorrhizal fungi; ectomycorrhizal fungi (associated with trees in boreal and temperate forests) and other fungal groups are entirely excluded. The uncertainty map accompanying the paper identifies specific regions, parts of Africa, Siberia, and South America, where sampling remains too sparse for reliable predictions. And the “68 quadrillion miles” figure represents total hyphal length if laid end-to-end, not the spatial footprint of the networks themselves, which is a distinct and much smaller measure.
Source: Stewart et al. (2026) “Global density and biomass of arbuscular mycorrhizal fungal networks.” Science 392(6803), pp. 1171-1176. DOI: 10.1126/science.adu4373. Data and code: Dryad DOI: 10.5061/dryad.p2ngf1w1f.