Moulds, mushrooms, lichen and toadstools. What would we do without them? We wouldn't survive. They are a very important part of all ecosystems, breaking down dead organic compounds and recycling them into useful loams etc for the rest of he ecology to use. One creature's trash is another's treasure. Plants couldn't absorb food without their symbiotic relationship with mycelia (which lives on and in their root system).
Mycelia are the "root" system of fungal life. We often see the mushrooms etc, which is actually just the sex organ of the fungus, but the most important part is often hidden or ignored, and that is the mycelium.
The biggest living organism is actually 2400 acres of a single mycelia in the American state of Oregon. It has been broken up by logging tracks, but it is still the same organism. It would have to have started from a single spore over a thousand years ago.
Because mycelia have the crucial task of breaking down organic compounds, it has been cited for potentially consuming pollution and toxins that have wrecked havoc on our delicate ecosystem (such as petroleum, or residual fertilisers). Mycelium does release carbons back into the air, but it is worth it for the cleansing and feeding effect it has on the environment. Some farmers actually add mycelia into their soil to boost plant growth.
It has already provides all sorts of things for us besides mushrooms. Penicillin and cheeses both use various mycelium. The original fungus that produces penicillin sold for a very tidy sum for the day, but it's value in lives saved and quality of lives improved is immeasurable.
The pattern created in fairy rings comes from the way mycelium operate underground, spreading outwards and consuming it's original base in the middle to recycle and reuse the parts that have run out of food. It will concentrate on areas where there is more organic compounds for it to eat, and less where there is less food, or where conditions are harsh. It will become hardened when there is not enough water.
In every breath each of us breathes there are usually more mycelia spores then pollen. It is a very diverse lifeform, giving the earth that wonderful smell after a rainfall. Some people are allergic to mould spores, just as some are allergic to pollen.
Although warm, nutritious, wet and not too bright places are there their favourite places, they are found in cooler places. Tropical forest fungi are an area which still needs a lot of research as a lot of fungi are still not classified. In fact, definitions between fungi are still unclear, as flagellated (spores with tails) mycelia and other lifeforms are still being classified, maybe even requiring a kingdom of their own.
As mycelia use enzymes to break down carbon based biomass, they are very interesting for other reasons, namely the production of chemicals that could be useful to us as medicines and other industries. It is an area that deserves funding from governments and industries simply as research for research sake, as it is highly likely that one would accidentally stumble upon some compound that is extremely useful (and therefore profitable) in some area of modern life.
Anyway, saving the world is all in a day's work for the humble mycelia. They deserve a pop song all of their own!