This investigation, we are looking at mycelium. Mycelium is a network of fungal threads or hyphae. They often grow underground but can also thrive in other places such as rotting tree trunks. Fungi often develop long threads which carry on subdividing. This can lead to an extensive underground network. That can be extremely useful to all kinds of plants and animals. A single spore can develop into a mycelium. The fruiting bodies of fungi, such as mushrooms, can sprout from a mycelium.