The “humongous fungus” are two different fungi—both enormous, underground fungus found in the United States in two different states. The first, the fungus Armillaria gallica, was found growing in 1992 in northern Michigan. It encompasses 35 acres (0.056 square miles, or 0.145 square kilometers) and is thought to be at least 1,500 years old. Also, in the Malheur National Forest in Oregon is a giant fungus of the same genus (different species), Armillaria ostoyae, or honey mushroom. This organism spans 2,200 acres (8.9 square kilometers)—making it, to date, the largest organism by area.

Scientists can prove that these huge organisms are a single fungus by taking samples of each fungus and performing DNA analyses on each sample. In both cases, it turns out that the genetic material of the respective samples were identical.