When you think of the biggest organisms on Earth, the blue whale probably springs to mind first. After all, these gargantuan beasts are up to 30 meters long (100 feet) and can weigh upward of 180 tons, meaning they’re probably bigger than even the largest dinosaurs. But it turns out that the world record holder for the largest living organism on Earth is something much less impressive to look at, but size wise it makes even the mighty blue whale seem puny; it’s a fungus.

More specifically, the contender for the world’s largest known organism is a honey fungus living in the Blue Mountains of Oregon. This humongous fungus occupies almost 2,400 acres (965 hectares) of soil, covering an area as big as 1,665 football fields.

Honey fungus, or honey mushroom, is actually the common name given to several different species of fungi in the genus Armillaria. These parasitic fungi colonize and kill a variety of trees and woody plants, a characteristic that has earned them the nickname “Gardeners Nightmare,” although some varieties are more benign than others.

Members of the Armillaria genus produce clumps of edible yellow-brown mushrooms that grow above ground. It is their color, rather than their flavor, which has earned them their nickname, although some argue they have a sweet aftertaste. These mushrooms are the fruiting bodies of much larger organisms that sprawl across significant areas underground. These mainly consist of flat, shoestring-like structures called rhizomorphs that search out new sources of food, and branching networks of tubular filaments called hyphae that collectively form a vegetative mass known as the mycelium.

Scientists first started to appreciate the sheer size of Armillaria after two massive specimens were discovered in 1992. The first was a 1,500 year old, 37-acre fungus (A. gallica) that lives in hardwood forests near Crystal Falls, Michigan, and the next (A. solidipes) covers an area of 1,500 acres (600 hectares) in southwestern Washington. But both of these pale in comparison to the A. solidipes specimen that was found a few years later.

The discovery was made after scientists began investigating why a significant number of trees were dying in a national forest in Oregon in 1998. Root samples were taken from 112 dead and dying trees, which revealed that all but four were infected with A. solidipes. Further examination revealed that 61 of the trees harbored specimens that were genetically identical, i.e. had originated from one particular organism. The greatest distance between infected trees was almost four kilometers (2.4 miles). Impressively, the fungus was found to cover an area of 9.6 square kilometers (3.7 square miles), and was estimated to be around 2,400 years old, although it could be as old as 8,650. This also makes it one of the oldest living organisms on Earth.

The discovery soon reignited the debate over what constitutes an individual organism, which has been defined as a group of genetically identical cells that can communicate and have a common purpose. Oregon’s gargantuan A. solidipes ticks these boxes, and so it currently holds the record as the largest known living organism.

[Via BBC, Scientific American and The Independent]