Ever see a set of mushrooms growing in a near-perfect circle? Or an arc of dead or dark grass on a green field? Folklore calls it the elf ring or fairy ring, but it actually has a very reasonable biological explanation.
Some European traditions connect elf rings with dancing witches, toads, or the Devil, but most popular seems to be the idea that this is where a ring of elves or fairies have been dancing. I think there’s a chapter about that in The Hobbit. Stepping into the circle is a bad idea: you may be trapped in a mad dance yourself, or be unable to escape the ring, or disappear entirely.
How do they form? Well, it seems pretty simple: a fungus grows, absorbs all of the nutrients around it, and expands outwards in all directions. As the centre of the circle is the origin of the fungus, it’s the first place that all the nutrients are depleted. Without food, the fungus stops growing in the centre, turning the circle into an expanding ring.
If the fungus is a mushroom, you get a nice ring of mushrooms. If the fungus is underground, you get a ring of dead or dark green grass (or, in the case of the image above, a dead circle of moss). This process can go on for quite some time: there’s a record of a 700-year-old ring in France that is more than half a kilometre in diameter.