Beneath the Blue Mountains of eastern Oregon, in the Malheur National Forest, a single honey fungus has been spreading through the roots of the forest for thousands of years. It now covers nearly ten square kilometres. Most of it is invisible. What you see at the surface is indirect: clusters of trees dying in the same way, and, for a few weeks in autumn, honey-coloured mushrooms pushing up through the soil. The organism itself is the network underneath.

It is one of the strongest contenders for the largest living thing on Earth. That phrasing is deliberate, and we will come back to it, because the title is more contested than the headlines suggest.

How it was found to be one thing

The fungus is Armillaria ostoyae, a honey fungus and a root pathogen, the kind of organism a forester first meets as a disease rather than a wonder. It was spotted in the Malheur in 1988 by a Forest Service worker, Greg Whipple, who put its extent at about 400 acres.

That turned out to be a large underestimate.

Establishing that the whole thing was a single individual took genetics. Researchers collected samples from across the affected area and tested whether they belonged to the same organism, partly by growing pairs of isolates together to see whether they fused or rejected each other, and partly by DNA testing. In a 2003 paper in the Canadian Journal of Forest Research, a Forest Service team (Ferguson, Dreisbach, Parks, Filip and Schmitt) mapped the largest genet, labelled Genet D, at about 2,385 acres, which is 3.7 square miles or roughly 9.6 square kilometres, and showed it was genetically distinct from neighbouring Armillaria individuals.

The detail that tends to get lost is that this is not the only Armillaria in the forest. According to the Oregon Encyclopedia, the famous one is the largest of several separate, genetically distinct patches of the same species in that part of the Malheur. What makes it notable is that this one patch is all a single organism.

What it is underground

Below the surface, the fungus exists mainly as mycelium, a mat of fine threads called hyphae that spread through soil and wood. Armillaria also grows rhizomorphs, tougher root-like strands sometimes called shoestrings, that let it travel from one tree root to the next. That is how it advances: root to root, tree to tree, fanning outward through the forest floor.

It feeds by killing.

The fungus attacks the roots and lower trunks of trees, and the dead and dying trees scattered across its territory are, in effect, a map of where it has been and where it is working now. The autumn mushrooms are the only part built to be seen, and only briefly. They are the fruiting bodies, the structures that release spores, and they give the honey fungus its name.

How old, and how the number is reached

The age figures are striking, and should be read as estimates rather than dates. Nobody has counted rings or carbon-dated the whole organism. Instead, knowing roughly how fast Armillaria spreads, between about 20 centimetres and a metre a year, you can work backward from its current size to how long it must have taken to get there.

That calculation gives a wide range. The fungus is usually put somewhere between about 1,900 and 8,650 years old, depending on the assumed spread rate. Even the low end makes it ancient, and the high end would place it among the oldest living things known. The spread rate is an average, though, and growth is not perfectly steady, so the range is genuinely uncertain rather than falsely precise.

Its weight is rougher still. You cannot put a forest-sized fungus on a scale, so the figures, which run from several thousand tonnes to perhaps 35,000, are extrapolations from how much fungal material tends to be present in infected soil and wood. They are best treated as order-of-magnitude guesses.

Largest of what, exactly

This is the part the headlines tend to flatten. The fungus is often called, plainly, the largest living organism on Earth. The more careful version is that it is one of a small group of contenders, and which one wins depends entirely on how you measure.

By area, it may no longer be the leader. In 2022, researchers reported that a single seagrass clone in Shark Bay, off Western Australia, covered roughly 200 square kilometres, though later summaries describe the surviving meadow as more than 180 square kilometres. By mass, a quaking aspen clone in Utah called Pando, a stand of genetically identical trees sharing one root system, is often estimated at around 6,000 tonnes and is commonly described as the heaviest known organism. The Oregon fungus remains one of the largest single organisms by area and may rival the others by mass, but the exact ranking depends on definitions and estimates.

Underneath the rankings sits an older question, which is what counts as one organism at all. The case for the fungus, the seagrass and the aspen standing on the same footing is that each is a single genet: one set of genetically identical cells, physically connected, spreading by cloning rather than by producing separate offspring. Accept that definition, and all three count as single individuals of extraordinary size.

What is not in question

The thing itself is not in doubt. Somewhere under the Malheur, one organism has been growing outward from a single point for at least nineteen hundred years, killing trees as it goes, surfacing for a few weeks each autumn, and otherwise keeping to the dark. The argument over its title is really an argument about words. The fungus does not depend on winning it.