Trees all over the world partner up with underground fungi, giving them the carbon they pull from the atmosphere in exchange for hard-to-get nutrients like nitrogen and phosphorous. Now, it turns out that these mycorrhizal fungi (like the amethyst deceiver, whose above-ground ”fruiting body” is shown in the image above) form an underground pipeline that shuttles that carbon between trees, even those of different species. The discovery paints a new picture of forests as far more connected than previously thought. Using a 45-meter-tall crane that towered over the Swiss forest canopy, researchers spent 5 years pumping carbon dioxide over five Norway spruce trees (Picea abies). The gas contained a specific blend of isotopes that let the researchers track the carbon as it moved its way through the trees—and through the fungal network that connects them to their neighbors. The authors estimate that 40% of the carbon in a tree’s fine roots comes from its neighbors, in a paper published today in Science. Some even makes its way up the neighbors’ trunks, where it’s used to build structures like bark. Altogether, the researchers estimate that in each hectare of forest, an average of 280 kilograms of carbon is exchanged between trees each year—nearly 4% of all of the carbon that those trees pull annually from the atmosphere. This hidden carbon superhighway could be key to forests’ resilience through droughts and other damaging events.