Beneath the soil in the Malheur National Forest in eastern Oregon, the United States, lurks a very large fungus that has been slowly weaving its way through the roots of trees for centuries to become the single largest living organism known to humans.

The fungus, Armillaria solidipes, remains mostly underground, hidden from sight, but every autumn just after the rains it sends up clusters of small yellow-brown mushrooms from the bases of trees it has infected. These mushrooms, commonly called “honey mushrooms”, are the most visible part of the fungus seen by the casual observer. The bulk of the fungus lies underneath the forest floor—a vast network of black filament-like structures called rhizomorphs, that creep through the soil, feeling out new root systems to colonize. The underground growth can stretch up to several square kilometers. The specimen in Malheur National Forest covers 2,200 acres (8.9 square kilometer), and has been named the “Humongous Fungus”. Another specimen, also named the “Humongous Fungus”, resides in a forest in Michigan’s Upper Peninsula. That one is spread over 37 acres.

The honey mushroom is the above-ground part of a vast subterranean fungus. Photo credit: Dan Molter/Wikimedia