When first discovered, the Armillaria gallica mushroom was thought to be about 110 tons, but it actually weighs roughly four times that much— and is a millennium older.
Wikimedia CommonsHoney mushrooms of the gargantuan Armillaria gallica.
When people think of the Earth’s largest organisms, they usually imagine whales, elephants, or even dinosaurs. But some of the largest organisms on Earth aren’t animals at all, but a sprawling fungi dubbed “humongous fungus.”
Armillaria fungi, known as “honey mushrooms,” is a unique species of fungus capable of growing to jaw-dropping sizes both underground and on decaying stumps or logs. The original “humongous fungus,” located in Michigan, has grown across such an impressive area over the last several thousand years that today it’s roughly the size of Vatican City.
But while the humongous fungus of Michigan was long thought to be the largest humongous fungus in the world, scientists now know that there are even larger species of Armillaria out there.
The Discovery Of The Original Humongous Fungus In Michigan
Lairich RigArmillaria rhizomorphs.
Scientists came across the original humongous fungus in Crystal Falls, Michigan entirely by accident. As local media in Detroit reported in 2018, the sprawling fungus was discovered during a Department of Defense project studying the properties of Extremely Low Frequency (ELF) radio waves.
Researchers wanted to determine if these frequencies could be used to communicate with submerged submarines. Because of the public concern about the effects these radio waves might have on the environment, a study was launched to study the effect of EFL radio waves on trees and plants.
And during the course of this study in Crystal Falls, researchers began to notice a fungus, now known as Armillaria gallica, eating some of the trees. As they observed the fungus, they began to realize that it was big — very big. They ultimately determined that the fungus covered at least 15 hectares (37 acres) and weighed about 22,000 pounds. It was able to grow so big and heavy, they said, because it was roughly 1,500 years old.
“This is the first report estimating the minimum size, mass, and age of an unambiguously defined fungal individual,” the researchers wrote in their 1992 study on the fungus. “Although the number of observations for plants and animals is much greater, members of the fungal kingdom should now be recognized as among the oldest and largest organisms on earth.”
The story gained media traction, and the press dubbed the discovery the “humongous fungus.” The nickname stuck. But Michigan’s humongous fungus was actually even bigger, and older, than the researchers thought.
But The Humongous Fungus Was Larger Than The Researchers Thought
Wikimedia CommonsFrom above ground, the humongous fungus looks like multiple organisms.
In 2018, researchers returned to Crystal Falls, Michigan to study the humongous fungus anew. They took 245 samples of the fungus, and confirmed through DNA analysis that the sprawling honey mushroom was the same individual organism. They found that it had been growing far longer than previously estimated, for about 2,500 years.
What’s more, they established that Crystal Falls’ Armillaria gallica was even larger than they had previously thought. It was not 37 acres, but 90. That meant that the humongous fungus covered an area double the size of New York City’s Grand Central Station.
And with the fungus’ larger size came a greater weight. The researchers determined that the fungus weighed about 440 tons, or about the size of three blue whales, the largest animals in the world.
This humongous fungus lives underground and reveals itself above ground in the form of clumps of honey mushrooms. The bulk of its 440-ton body exists within the soil in the form of rhizomorphs, which are long, black tendrils that extend not unlike roots throughout the ground.
Igor PavlovA forest of trees dying due to damage from Armillaria.
Rhizomorphs can grow for miles through the soil, fueled by a diet of wood, and are the reason why Armillaria has reached such massive proportions.
As cool as these humongous fungi are, their impressive size comes at a high price. These mushrooms are parasitic and nestle their way underneath the bark of trees, eat the wood, and kill them. The fungus lives underground but in many places with large Armillaria, the devastation can be seen from above.
The Humongous Fungus Is Now A Unique Michigan Attraction, But It Isn’t The Largest Armillaria In The World
Since it’s discovery in the 1990s, Michigan has capitalized on the unique, natural attraction. The town of Crystal Falls holds a “Humongous Fungus Festival” each August.
The festival includes an array of fungus or mushroom themes activities. They also have an annual parade and t-shirt design contest. Needless to say, locals have embraced the humongous fungus.
Humongous Fungus Festival/FacebookPeople dressed up for the 2025 Humongous Fungus Festival in Crystal Falls, Michigan.
While this organism is huge, probably one of the largest in the world, it is not the largest of the Armillaria mushrooms. That title goes to an Armillaria ostoyae fungus which covers 2,200 acres in Oregon’s Blue Mountains.
It’s discovery was the result of a worldwide search for the world’s largest fungus, set-off by the discovery of the humongous fungus in Crystal Falls, Michigan. Similarly to its midwestern cousin, the Armillaria ostoyae grows rhizomorphs underground, infecting and paratize tree roots, eventually killing the tree.
Wikimedia CommonsArmillaria Gallica growing on a tree trunk.
And, for those of you who are curious, you can techinically eat the honey mushrooms of the Armillaria ostoyae. But, they do need to be very well cooked in order to avoid stomach problems.
While these organism are certainly huge, there have been some debate if they can be categorized in the same way other large species like blue whales or red wood trees are. In a book published a few years after the 1992 Armillaria gallica paper, author Jack Wilson questions what really makes an organism biologically “independent.”
No matter the technical or scientific categorizations of these impressively large fungus, people still love them. There is certainly some formidable fungus among us.
After learning about the gigantic Armillaria gallica mushroom, check out these fascinating photographs of Cordyceps, the killer fungus that takes an insect host. Then, take a look at this horrifying fungus which uses mind control on insects to make zombie ants.
