Hank Green:  A few weeks ago I was walking through the park and I came across something kind of weird, something that I could not immediately explain.  And when that happens, it's usually pretty annoying for me.  There was an arc in the grass of greener grass about 10 feet long, as if someone had very carefully spread fertilizer along a curving stripe of the park.  And once I saw it, I started to see more.  Dozens of them spread across the park, one of them was even a full circle.

When I find something just hanging out in the park that I've never heard of and I can't explain, that's frustrating, but thankfully I have Google and SciShow as a lovely, helpful audience because now I have the answer:  Fairy rings.

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Even weirder:  A perfect circle of mushrooms.  before we had science, people thought that these might be portals between worlds.  If some weird nuisance was occurring in the village, it was likely caused by elves or fairies coming into our world through fairy rings.  But alas, this is not the case.  I've already given you a hint as to what they actually are.

The key players here are not some elven mischief makers, but fungi.  The most common fairy ring mushroom is the Scotch Bonnet and like most fungi, the bulk of its body is its mycelium and it's underground.

The mycelium is kind of like a subterranean tree with thousands of fine, branch-like structures that grow outward in a circle.  As it expands underground, the mycelium releases chemicals that break down organic matter in the soil nearby so it has food by the time it grows there.  And since by doing that it's basically fertilizing the soil as it goes, the grass above the leading edge of the mycelium, for a short time, has more nutrients available than usual which creates rings of rich, thick, green grass.

When the mycelium reaches the enriched soil though, the party is over because the fungus then absorbs all of the nutrients out of the soil and the grass above it dies.  And it's at this point, as the fungus matures, that you see the stuff that you probably think of when you think of fungus.  Mushroom begin to sprout, which are actually known as the fungus's fruiting bodies, they're like the fruits of the tree.  They generate and release spores that the fungus uses to reproduce.

In some cases, unless there's some physical barrier to stop it, a fairy ring can grow for years and cover huge areas.  One fairy ring in France was measured to be some 600 meters wide.

It's important to note now that fairy rings can also be used to describe a different phenomenon.  Redwood groves can also grow in something called a fairy ring.  They're completely different; caused when shoot sprout up from the base of a fallen tree

In the deserts of Namibia, meanwhile, grasses have been found to grow in rings caused by sand termites that live underground.  The termites spread out in a circle, eating the roots of all the grasses they come across.  This helps the soil retain water, which nourishes the grass at the perimeter of the circle.

Sometimes it can feel like science is poo pooing the mystical and the fantastical but in my opinion, there's more depth of beauty in the fantasy of the real.  That giant fungi reach in circles underground with 600 meter diameters converting detritus into fertilizer, forming themselves, altering their environment, and interacting on a global scale to form part of this beautiful and complex web of life.

But you are entitled to your own opinion as well.

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