Mycelium is how fungi feed.Plants do photosynthesis to make their own food.Most animals ,find food in the world and put it inside their bodies ,where it is digested and absorbed.Fungi however have different strategy, they digest the world where it is and absorb it into their bodies.Their hyphae are long and branched, and only a single cell thick-between two and twenty micrometers in diameter, over 5 times thinner than an average human hair.
The difference between fungi and animals is simple:animals put food in their bodies, whereas fungi put their bodies in the food.
No matter where fungi grow, they must be able to insinuate themselves within their source of food.Todo so they use pressure.Cases where mycelium has to break through particularly tough barriers, as disease causing fungi down infecting plants, they develop special penentrative hyphae which can reach pressures of 50 to 80 atmospheres
One study estimated that if a hyphae was as wide as a human handset would be able to lift an eight tonne school bus!!