For a long time the mushroom was the fungus, the way an apple is taken for the tree. The mushroom, though, is only the fruit—a brief, showy organ pushed up to release spores. The organism itself lives underground as mycelium, a lace of threads finer than hair that can spread for acres and persist for centuries. Botanists once filed fungi with plants, misled by their stillness and their roots-that-are-not-roots. We now know a fungus is closer kin to an animal than to the oak it embraces. What looks like a passive web is in fact a slow forager, dissolving rock and wood, trading sugars with tree roots along channels that some researchers, perhaps too eagerly, have called a forest's nervous system. The metaphor flatters us. A mycelial network does not think. But it does distribute, remember which partners paid and which cheated, and route resources accordingly—which is more than we once granted a thing without a face.