Reduce the number of words, making it concise yet still stating the valid points: Mycelium is the vegetative structure of a fungus, comprising a network of hyphae (filaments) that can grow underground, in soil, or on various substrates. It is often like

Mycelium is the vegetative part of fungi, made up of hyphae that grow underground or on substrates, similar to plant roots. It absorbs nutrients and water, facilitates decomposition, and recycles nutrients. Although usually unseen, mycelium can produce fruiting bodies like mushrooms under the right conditions.