The World’s Largest Organism Is a 35,000-Ton Fungus – At Least, For Now

• The largest organism is the Armillaria ostoyae, also known as the “humongous fungus.”

• The fungus covers 2,385 acres and is 8,650 years old, weighing 35,000 tons.

• Experts suggest that other organisms like the Antarctic blue whale, the 106-acre aspen forest, or two closely related hybridized eelgrass species could compete for the world’s largest organism.

Quarter-sized white button mushrooms, which are typically found on pizza slices and in grocery produce aisles, are close relatives of what some consider the world’s largest organism. Their cousin? A single, massive organism, Armillaria ostoyae, is located in Oregon's Malheur National Forest.

Also known as the “humongous fungus,” many experts consider this vast honey mushroom network the world’s largest organism.

Antonis Rokas, a professor of biological sciences at Vanderbilt University, says current estimates of the humongous fungus suggest it covers 2,385 acres, or nearly three times the size of Central Park. At some 8,650 years old, the fungus weighs approximately 35,000 tons. And that’s only when considering how much of the organism has been identified by experts.

However, estimating the fungus’ true size and whether it is indeed a single organism is complicated. That’s because many Armillaria species have indeterminate growth, meaning that, like many plant species, they will continue to grow so long as their conditions permit, expanding to the capacity of their environment. On the other hand, humans and other mammal species have a predetermined size that reaches its full potential at maturity.

Mushrooms, the fruiting structure of certain fungi species, are visible only above the soil's surface. Mushrooms release spores, like certain plants release seeds, as a form of sexual reproduction. Underneath the soil's surface, however, the fungus grows a “vast network of threadlike structures” known as the mycelium.

“You can think of the humungous fungus as this underground network of an organism; it doesn’t have a very defined shape other than its network of hairlike structures that spread over a large area. That characteristic, mycelial growth, is a feature of many fungi,” says Rokas. “What makes this particular organism somewhat special is that it’s a big generalist, so it can grow on various substrates.”

Chain-like mycelium allows fungi to interact with the natural world on a cellular and molecular level, making them efficient at exploring microscopic places and finding otherwise difficult-to-access nutrients. Oregon’s moist Pacific Northwestern climate is ripe with dead and decomposing wood and plant material, providing ample space and food for the humongous fungus to continue its growth at an indeterminate rate.

“As a result, they are known as the most efficient decomposers of terrestrial plant matter in the world. This helps them break down dead plants and recycle their nutrients back into the ecosystem so other organisms can access this energy,” says Andrew Wilson, associate curator of mycology at the Denver Botanic Gardens.

Armillaria is considered both an infecting pathogen and a saprotroph, an organism that feeds on nonliving organic matter known as detritus. While fungi like the humongous fungus can be an essential decomposer of dead wood, they can also attack partially dead trees, even killing them to create more food sources.

For the humongous fungus to grow to such a size, it needs enough host trees susceptible to Armillaria to grow near with the right conditions and limited competition from other fungi and organisms.

Read More: 5 Of The Biggest Animals To Ever Live On Earth

But because so much of the humongous fungus is below ground, accurately estimating its true size is an arduous task. Adding in the consideration of what constitutes an organism makes this award even more complicated.

Leigh Torres of the Oregon State University Marine Mammal Institute studies marine megafauna and the life cycles of the ocean’s largest organisms, including Antarctic blue whales, which many consider the world’s largest animal.

Before the whaling era, Antarctic blue whales could reach up to 100 feet long, but today they are typically around 85 feet. With a mouth that measures one-third of its body length, the Antarctic blue whale uses baleen to sieve massive quantities of zooplankton and other tiny marine organisms from the surrounding ocean water.

Then again, some experts contend that a 106-acre aspen forest in central Utah could be the world’s largest organism. According to the Western Aspen Alliance at Utah State University, the “Pando” aspen, which means “I spread” in Latin, consists of 47,000 trees weighing a collective 13 billion pounds. Each individual stem is genetically identical and connected by a single underground root system, making the entirety of the forest one organism.

In 2022, Australian researchers reported finding a new potential contender for the world’s largest organism: two closely related hybridized eelgrass species in Western Australia’s Shark Bay. Here, genetic tests revealed that a 111-mile-wide meadow of shallow seagrass stemmed from a single hybrid plant that had initially been cloned some 4,500 years ago. That same year, Guinness World Records awarded the seagrass the “world’s largest organism” accolade.

Read More: 5 Unusually Large Animals You Probably Haven’t Heard Of

However, organisms grow, change, and evolve over time, and estimating an organism’s true size is, to some degree, a philosophical question of where one individual ends and another begins.

Aspen, the humongous fungus and the shark bay seagrass hybrid species are tied to clonal growth, a type of asexual reproduction where one organism can produce genetically identical offspring that remain connected and share resources. A mushroom like the humongous fungus comprises many related individuals who share identical genes from a single individual whose spores have self-replicated, or inbred.

But experts like Wilson and Rokas say that this concept of clonal growth begs the question of individualism and what separates one organism from another. Whether millions of clones constitute one collective organism, or many individuals depends on who you ask.

“This gets down to the philosophical discussions about the nature of life,” says Wilson. An Antarctic blue whale, for example, is a sentient being with its own thoughts and emotions. Can a fungus or plant say the same?

“The humongous fungus will always be a mystery because you can never really measure its full extent. It’s the nature of fungus to be cryptic and to keep their secret – but that’s also what’s fun about them!” says Wilson.

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Proceedings of the Royal Society B. Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment

• Guinness Book of World Records. Largest living organism

Madison’s reporting focuses on marine and environmental issues, climate change, and novel scientific discoveries related to health and technology. Raised on an island in southeast Alaska, Madison is now based in western Montana. Her writing has been featured in Time, Snopes, Business Insider, Mountain Journal, EcoWatch, and Alaska Magazine, among others. When not writing, Madison teaches yoga, raises chickens, and fosters adoptable dogs and cats.