Beneath the ground, vast networks of fungi quietly support plant life and help regulate the planet's climate by moving carbon into soils. Now, researchers have created the first global maps showing where these underground fungal networks are found and how much of them exist worldwide.
The study, published in Science, focuses on arbuscular mycorrhizal fungi, a group of fungi that form partnerships with most plants on Earth. Alongside the research, scientists released an interactive visualization that lets users explore the scale of this hidden underground infrastructure, helping identify where these fungal networks are thriving and where they may be under threat.
Arbuscular mycorrhizal fungi, commonly called AM fungi, form mutually beneficial relationships with approximately 70 percent of plant species worldwide. Plants provide the fungi with carbon produced through photosynthesis, while the fungi supply plants with nutrients and water. These underground networks function as living infrastructure that helps sustain ecosystems and move carbon into the ground. Until now, scientists had not attempted to estimate and map the physical density and worldwide distribution of these networks.
To build the new maps, researchers compiled measurements from more than 16,000 soil cores collected around the world, then used machine-learning models incorporating environmental data from deserts, tundra, forests, and other ecosystems to predict fungal network density where direct measurements were unavailable. The team also used robotic imaging to analyze more than 300,000 living AM fungal hyphae grown in the lab, allowing researchers to estimate the total length and mass of the global network.
Their analysis suggests that AM fungal networks extend for approximately 110 quadrillion kilometers and contain roughly 300 megatons of carbon, four to six times the mass of all living humans.
"It is hard to overstate the importance and enormity of these fungi," said lead author Dr. Justin Stewart. "There could be up to 10 meters of mycorrhizal network in just a teaspoon of soil."
Scientists often describe mycorrhizal networks as one of Earth's circulatory systems, transporting carbon, nutrients, and water throughout underground ecosystems. In healthy soils, these networks can expand the foraging area of plant roots by up to 100 times and provide more than 80 percent of a plant's phosphorous needs.
The researchers created a detailed global map of this fungal infrastructure, with estimates calculated for every square kilometer of terrestrial land, excluding ice caps and regions where data were insufficient. The data behind the maps are now publicly available, giving decision-makers new tools for monitoring underground fungal communities.
The work builds on earlier research showing how mycorrhizal fungi and plants exchange carbon and nutrients, with carbon moving through these networks at speeds reaching 120 micrometers per second. The new study extends that work to a planetary scale.
The researchers also identified areas of concern. Network densities in croplands are predicted to be about half those found in wild ecosystems, while wild grasslands contain roughly 40 percent of the world's arbuscular mycorrhizal biomass. Despite their importance, grasslands remain among the least protected ecosystems on Earth and are being converted to farmland four times faster than forests. These findings echo earlier research showing that most biodiversity hotspots for these fungi lie outside protected areas.
For evolutionary biologist Dr. Toby Kiers, the evidence highlights the need to include fungi in climate and conservation planning. "Fungi have been ignored in climate and conservation for too long," she said. "Now is the time to change that trajectory."
While the new maps reveal the extraordinary scale of Earth's underground fungal networks, they highlight major gaps in scientific knowledge, with large regions of the world still unsampled.