If you walk through a forest in Oregon, you are likely to see the Honey Mushroom, Armillaria. From the sight of it, you might judge this organism to be around half a foot long. In reality, it is likely over 3 and a half miles long. That is because the part of the organism you actually see is less than the tip of the iceberg: most of it is underground. The actual organism is mycelium, a vast network of branching, interconnected fibers. Mushrooms are the fruit of mycelium, just like an apple is the fruit of an apple tree.
The largest patch of mycelium was found in Malheur National Forest in the Blue Mountains of eastern Oregon, and may in fact be the largest organism in the world. It covers over 2,200 acres, or enough to fill 1,600 football fields. It is estimated to be around 2,400 years old.
Source: Simple Life Forms