What is probably the largest living organism on earth has been discovered in the Malheur National Forest in eastern Oregon: a fungus which is living three feet underground and is estimated to cover 2,200 acres. Officially known as Armillaria ostoyae, or honey mushroom, the fungus is 3.5 miles across and takes up to 1,665 football fields. Experts estimate that the giant mushroom is at least 2,400 years old.