Scientists used machine learning and a high‑resolution imaging robot to map the planet’s hidden fungal circulatory system. They estimate the total length of arbuscular mycorrhizal hyphae at 68 quadrillion miles, a scale that would stretch around the Sun 730 million times. The filaments store roughly 300 megatons of carbon, dwarfing the carbon held by all humans.

The team compiled data from more than 16,000 soil samples, extracting hyphal density and filament width. Training a machine learning model on this dataset allowed prediction of fungal abundance across unsampled regions. The resulting global heat map shows grasslands as hotspots, with especially dense networks in the Florida Everglades, South Sudan’s Sudd wetland, and the Tibetan steppe.

Findings reveal that cultivated soils contain about half the fungal density of natural lands, suggesting current farming practices may erode a major carbon sink. Policymakers and carbon‑credit firms now have a concrete metric to assess ecosystem‑based offsets. Protecting these underground networks will be essential for meeting climate goals.