The part that you see is only a tiny bit of the entire organism, often less than 1%. Most of it is a network of thin strands called mycelium, that spread through the soil, decaying wood, and other organic material. Some of these mycelium mats are huge. The largest known is in Malheur National Forest in Oregon. It is estimated to cover about 9 square kilometers, making it one of the largest living things on Earth.