One of several fairy rings in Lambrok Meadow, caused by the mycelium of a subterranean fungus.
The fungus secretes enzymes from the growing tips of the underground threads, called hyphae, that make up the mycelium. The enzymes break down the nutrients in the soil into molecules small enough to be absorbed through the walls of the hyphae. The mycelium grows outward, looking for more nutrient-rich soil, while in its now nutrient-deficient centre, the fungus dies off and eventually an expanding ring is formed.
Oyster mushroom mycelium  showing radiating growth pattern of the hyphae and  the fruiting bodies of a fairy ring mushroom (Marasmius oreades).
Sometimes, the grass inside a fairy ring will be weakened or even die but the Lambrok Meadow fungus must be of the type that produces hormone-like chemicals called gibberellins, which alter the grass’s growth patterns, causing this luxurious dark green ring.
There are two types of fairy ring fungus. Those found in the woods are called tethered, because they are formed by mycorrhizal fungi living in symbiosis with trees. Meadow fairy rings are called free, because they are not connected with other organisms. Free fairy ring fungi are saprotrophic: they grow on rotting material.
There are about 60 species of fungi which can grow in this fairy ring pattern. The most frequently found is Marasmius oreades, which goes by several common names: fairy ring champignons, fairy ring mushrooms or Scotch bonnets. We are unlikely to be able to identify this one, though, until it develops fruiting bodies next autumn.