Mycelium can become visible to the naked eye, for example, on various surfaces and substrates, such as damp walls, spoiled food, etc.

Mycelia, grown on solid surfaces, are called colonies. Some individual colonies can reach extraordinary sizes and ages, as in the case of Armillaria ostoyae (a tree pathogenic fungus), which extends over an area of over 900 ha, with an estimated age of almost 9000 years.