Yet one surprisingly basic question has remained unanswered until now: just how much of this hidden network is buried underground? The answer, fresh from a new study by the Society for the Protection of Underground Networks (SPUN), is: a hell of a lot. Roots and allScientists often call these fungal networks Earth's circulatory systems, and it’s easy to see why. As plants pull carbon dioxide from the atmosphere, they send some of that carbon underground to their fungal partners, helping store it in the soil. Yet while we devote huge resources to planting and protecting forests, the vast underground network where much of that carbon ends up has received comparatively little attention.