The Largest Living Organism Is a Fungus

While the towering blue whale holds the record for the largest animal, the most immense known living thing on Earth is a surprisingly unassuming fungus. In Oregon's Malheur National Forest, a single organism of the honey fungus species, *Armillaria ostoyae*, sprawls across an astonishing 3.7 square miles. What is visible above ground are merely small, innocent-looking honey mushrooms that sprout in the fall. The vast majority of this behemoth exists as an intricate underground network (Review) of mycelial cords, often called rhizomorphs or "shoestrings," that weave through the soil and infiltrate tree roots.

This colossal fungus is a powerful and persistent forest pathogen. Its rhizomorphs act as relentless agents of expansion, spreading from tree to tree and causing a fatal root disease. As the fungus grows, it secretes digestive enzymes to break down wood for nutrients, slowly killing its host trees over decades. Scientists were able to determine that this was a single, continuous organism by taking samples from various locations across the forest and finding them to be genetically identical. By analyzing its slow growth rate of just one to three feet per year, researchers have estimated its age to be at least 2,400 years, and possibly as old as 8,650, making it one of the planet's oldest living entities as well.

The title of "largest organism" can be a matter of debate depending on the metric used, with contenders like the massive Pando aspen colony in Utah, which wins for sheer weight. However, by area, the "Humongous Fungus" of Oregon remains a top contender, a silent, slow-moving giant that plays a significant, if destructive, role in the life and death of the forest it inhabits. Its discovery in 1998 revealed a hidden, interconnected world beneath our feet, fundamentally changing our understanding of the scale at which a single life form can exist.