It’s a sunny July day during an otherwise exceptionally rainy season in the lush green mountains of Huila, in Colombia’s eastern Andes. Adriana Corrales, her assistant and a local guide climb through the dense cloud forest. Above them, birds sing and monkeys howl through the canopy of ancient Colombian black oaks (Trigonobalanus excelsa), an endangered tree species. But the researchers keep their eyes on the ground.

“All this forest above us, and we are here looking down,” says Corrales, a fungi ecologist and expedition leader at the Society for the Protection of Underground Networks (SPUN), a nonprofit research organization mapping fungi worldwide. For the last two years, the group has been searching in Colombia’s black oak forests for mycorrhizae, a type of fungi that establishes a unique symbiosis with plants that’s fundamental to keeping forests alive.

Most plants worldwide are associated with these fungi. Mycorrhizae grow around roots, forming vast networks of thin, cotton-like filaments that extend into the lower soil levels and reach the litter fall. Through this system, the fungi can break down organic matter, such as dry leaves, and even mine minerals in rocks and deliver water and essential nutrients directly to plants’ roots. In return, the roots provide the fungi with sugars, essential for their survival.