The Largest Living Organism on Earth Is a Fungus in Oregon the Size of a Small City

A Monster Hidden Underground

In the Malheur National Forest in Oregon's Blue Mountains, there appears to be nothing particularly unusual. The forest looks like any other — ponderosa pines, firs, and the occasional gap where a tree has died. But beneath the surface, spread through the soil across an area larger than 1,600 football fields, lives a single organism that dwarfs every whale, elephant, and tree on Earth. It is a honey fungus, Armillaria ostoyae, and it covers more than 9.65 square kilometers — an area of about 3.7 square miles — with a network of thread-like fungal strands called rhizomorphs. It is commonly referred to as the Humongous Fungus.

This organism was identified in a 1992 study published in the journal Nature, which used DNA analysis to confirm that what appeared to be thousands of separate fungal colonies across the forest were in fact genetically identical — a single individual. The research team collected samples from across the affected area and found that all the material shared the same genetic fingerprint, meaning it all originated from a single founding spore that began growing thousands of years ago.

What Makes It One Organism

Fungi present a genuine challenge to the intuitive notion of what an "organism" is. Unlike an animal or a plant, fungi consist largely of mycelia — branching networks of microscopic filaments called hyphae that spread through soil or wood. The visible mushrooms that appear aboveground in autumn are not the organism itself but its reproductive structures, briefly deployed to disperse spores before withering away. The organism is the mycelium, and in the case of the Humongous Fungus, the mycelium is vast.

Genetic identity is the key criterion: if all the mycelial tissue in that 9-kilometer area shares the same genome and is physically interconnected, it counts as a single individual. The Oregon fungus meets both criteria. It is not just genetically identical throughout but physically connected — nutrients, signaling molecules, and water can move through the network from one end to the other.

A Slow-Motion Predator

Armillaria ostoyae is a pathogen. It feeds on living and dead trees, sending its rhizomorphs — thick, root-like strands of bundled hyphae — beneath the bark of tree roots, where they grow and digest the woody tissue. An infected tree first shows signs of stress: thinning foliage, reduced growth. Eventually the fungal mat kills the cambium layer, girdling the tree and cutting off nutrient flow. The fungus then continues into the dead wood, completing the decomposition.

The damage caused by the Humongous Fungus in the Malheur National Forest is extensive. Foresters have estimated that it has killed thousands of trees over its lifetime. The death of trees creates openings in the forest canopy that fill with competing vegetation, reshaping the structure of the ecosystem. From the fungus's perspective, this is simply feeding — it has been doing so for an estimated 2,400 to 8,650 years, with estimates varying depending on the method used to assess the fungus's growth rate and area.

Why Biology Needed a New Concept of Size

Before the discovery of the Humongous Fungus, size records in biology were typically awarded to large, visually impressive organisms — blue whales, giant sequoias, large coral colonies. The Oregon fungus forced a more rigorous examination of what it means to be "large." Is an organism defined by the volume of its body? Its mass? The area it occupies? Its genetic continuity?

In the case of the fungus, area and genetic identity are the most meaningful measures. By those criteria, it remains the undisputed record holder among all known living things, a reminder that the most significant organisms on Earth are not always the ones you can see.