In 1998 a Forest Service scientist named Catherine Parks went looking for whatever was killing the fir trees east of Prairie City, Oregon. She took root samples from 112 dying trees, grew the fungus from each one in a dish, and then set the cultures side by side in pairs. Two cultures of honey fungus from different individuals refuse each other: where they meet, a line forms between them. Two from the same individual simply grow together. Sixty-one of hers grew together, from trees a long way apart. The thing killing the forest was one fungus, Armillaria ostoyae, spread through about 2,385 acres of soil, nearly four square miles, and somewhere between 2,400 and 8,650 years old. It may be the largest living thing anyone has found.

Nearly all of it is underground, in dark threads that run through the soil from root to root. What shows above ground, each autumn, is a few clumps of ordinary brown mushrooms at the foot of a tree, the kind you would walk past. I like that the way to measure something too big to see was to take small pieces of it, two at a time, and ask whether they recognised each other.