Mushrooms are the reproductive organs of fungi. The rest of the fungus is underground or inside the organism the fungus thrives in.
I find that unlikely. There isn't anything beneath a mushroom if you pluck it.
Otherwise, mushrooms are the manliest things ever.
Below the mushroom is the so-called
mycelium, which is a network of long, thin threads of fungal cells. You can barely see them with the naked eye, but with a magnifying glass or a microscope you can.
A lot of fungi live in symbiosys with trees, effectively increasing the surface area of the tree roots with their long thin threads, thus helping the tree absorb water and nutrients in exchange for sugars.
Fungi stand somewhat between single- and multicellular organisms. The mycelium is much more of a colony of singlecellular organisms, yet once it accumulates enough mass, it can grow complex and specialized organs like mushrooms.
The 'witches circles' of mushrooms are not multiple organisms, but rather the reproductive organs (or flowers, if you like) of a complex of mycelium. The bigger the circle, the older the fungus (a fungus does not nescessarily die after sprouting mushrooms. Some mushroom circles are 100s of years old).
It once started as one tiny spore in the middle of that circle, and expands outwards each year.
The mycelium is the main body of the fungus. It grows mushrooms in a certain period of the year for reproduction.