Never even nibble on a mushroom if you re not absolutely sure what it is, Some can be quite poisonous. The mushrooms we see are only the spore producing part of the fungus, that's why they can grow so fast. The main part is either underground or in decaying wood depending on the type. A single underground fungus plant can inhabit an acre or more under the soil. Unlike green plants mushrooms are unable to make food from sunshine. They live by absorbing the food made by green plants when that plant dies and decays, releasing the stored nutrients.