The world's largest known organism is a single fungus spanning 3.7 square miles

In Oregon's Malheur National Forest, one genetic individual of the fungus Armillaria ostoyae has spread underground for an estimated 2,000-8,000 years, now covering roughly 2,385 acres. It spreads via black, root-like rhizomorphs that kill conifers as it grows, and DNA fingerprinting confirmed it's all one organism, not a cluster of separate ones.

— Bonnie A. Ferguson et al., Coarse-scale population structure of pathogenic Armillaria species in a mixed-conifer forest in the Blue Mountains of northeast Oregon — Canadian Journal of Forest Research, 2003