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

Myceliums grown on solid substrates are referred to as colonies. Some individual colonies can reach unusual sizes and ages, as in the case of Armillaria solidipes (a fungus pathogenic to trees), which extends over an area of more than 900 ha, with an estimated age of nearly 9000 years.