These subterranean structures, called arbuscular mycorrhizal fungal networks, work in partnership with most of the world's land plants, feeding plants nitrogen and phosphorus in return for their carbon. Now, the first global map of this fungal network has revealed where their intricate branching structures are most densely packed. In grasslands that are high-altitude or flooded grasslands, such as the Everglades in Florida, the top 6 inches (15 centimeters) of soil are especially dense, containing around 40% of the global fungal biomass. Arbuscular mycorrhizal fungi are made up of tiny branching threads called hyphae. These hyphal networks form two-way pipes to...