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Strange “Fairy Rings” in Seafloor Explained

Here’s one mystery that has come full circle: Scientists have figured out the meaning behind ocean “fairy rings.”

In 2008 a tourist photographed some bizarre circles on the seafloor in shallow waters of the Baltic Ocean off the chalky cliffs of Møn, Denmark.

Photo of the eelgrass circles in the shallow water off the coast of Denmark.
Circles seen in shallow water off the coast of Denmark. Photograph by Jacob T. Johansen

The images piqued public interest, with people offering intriguing guesses that included World War II bomb craters and alien crop circles. (Also see “New Theory Explains Africa’s Mysterious Fairy Circles.”)

Now scientists have an answer: Sulfide, a toxic substance that accumulates on the ocean bottom, is stunting vegetation called eelgrass, creating rings of healthier plants around these diseased zones.

Biologists Marianne Holmer at the University of Southern Denmark and Jens Borum at the University of Copenhagen studied samples from five of the circular patches, which ranged from 6.5 to 49 feet (2 to 15 meters) in diameter, as well as the mud accumulating among the eelgrass plants.

The team found that on the inner parts of the ring, the eelgrass roots and leaves were shorter, less dense, and overall less robust. They also found the mud was high in sulfide, a chemical compound of sulfur that’s released when plants die.

Sulfide usually bonds with iron, which is naturally found in the ocean. But in the chalky, iron-poor sediments in this area of Denmark, sulfide accumulates in the sediment, where it’s eventually taken up by older eelgrass plants, according to the study, published February 2014 in the journal Marine Biology.

Eelgrass grows radially, or out from a center point, so the older plants that are more exposed to the sulfide and thus weaker are in the middle, with healthier, younger plants around the perimeter—explaining the rings.

Ebbing Eelgrass 

Sulfide is also common in waters low in oxygen, which is increasingly a reality due to pollution. For instance, runoff from lawn fertilizers and industrial waste shuttles nutrients like phosphorus and nitrogen into the oceans, causing blooms of algae that eat up available oxygen in the water. Not only that, the algae also block the sun and kill seagrass. (Read more about ocean pollution.)

Photo of eelgrass growing in the shallow water off the coast of Denmark.
Eelgrass provides habitat for marine life. Photograph by Ole Pedersen

Due to these causes, as well as outright destruction, seagrasses are in decline the world over, the scientists noted.

That could be bad news for the ocean, since eelgrass is “a very important habitat for all kind of sea creatures,” noted Laura Murray, a marine ecologist at the University of Maryland Center for Environmental Science, who was not involved in the study.

“Smaller fish or molting crabs will hide in eelgrass to escape predators, and the grass will grow algae on its leaves that are eaten by other critters,” she said.

So eelgrass has “its own sort of food web right there.” (See pictures of marine species under threat.)

Eelgrass also protects shorelines from erosion and filters particles and sediments to help keep the water clear.

The University of Southern Denmark is a partner and coordinator of NOVA GRASS, a research project that’s working on restoring eelgrass meadows to their former lushness.

As for those rings, we finally have a perfectly good explanation—either that or those wily fairies have staged one awesome cover-up.

Follow Liz Langley on Facebook and Twitter.


  1. snehal
    March 22, 2014, 4:22 am

    this is amazing…………

  2. patty
    March 3, 2014, 6:14 pm

    My reply was to the stories about the stink bug.

  3. Original Hippie
    Minneapolis, Minnesota
    February 23, 2014, 5:01 pm

    Quote: “Eelgrass grows radially, or out from a center point…”
    Do you not think it would have been easier to say, “The eelgrass grows in a mandala”?

  4. Emmet Dunne
    February 22, 2014, 4:07 pm

    I’ve seen similar rings with mounds on land in Ireland when I was a kid and they gave them the same name “fairy rings”

  5. anonymous
    February 22, 2014, 10:09 am

    I do not understand the mushroom circles because I have not even seen one.

  6. Geytee Ara
    Islamabad, Pakistan
    February 22, 2014, 1:11 am

    So interesting to learn all this I have two questions if someone can help. fairy circles are also found in deserts why always in circles and are they found in every ocean bed and deserts or only in preferred zones.

  7. Phanupong Asvakiat
    February 21, 2014, 3:03 pm

    Zostera able to live marine live due to help of entocladia and DMSP break-down by marine fungus such as Fusarium……..Planet Earth(written through Planet human agent)

  8. Liz Langley
    February 21, 2014, 12:04 pm

    @Q The way eelgrass grows, it starts from a center point and radiates out in a circle – I think of it like those starburst fireworks that start at a center point and burst out from the middle. The older grass is at the center and it doesn’t stand up as well to the sulfides that accumulate in the chalky soil in these waters, so when the older grass in the center dies it leaves a ring of grass on the outside. Some mushrooms grow in a similar, circular pattern. The mushroom itself is the fruit of a fungus growing underground called mycelium which grows outward, from a center point, like eelgrass does. As it grows outward you can sometimes see a dark patch in the soil where the grass has died because the mycelium has taken up all the nutrients in the soil. The mushrooms will pop up on the edge of that mycelium circle, usually after it rains.

    🙂 I hope this helps! Thank you to everyone for questions!

  9. Sheila Simmons
    February 21, 2014, 11:25 am

    We get similar circles or almost in the grass near edge of L. Ont. which is on Limestone. I wonder if there are parallels?

  10. The Observer
    February 21, 2014, 11:17 am

    What about those crops circle on the land?? Are they also eelgrass?? I doubt so. On land the crop circles they could have been created by an alien group from the stars. No human can do it on an overnight basis. A test had shown radiation being detected on any new crops circle throughout the globe.
    It is a message to the humankind that we are not alone in the Universe.

  11. Lindsay
    Charleston, SC
    February 21, 2014, 9:33 am

    I agree with Jaden. I though fairy rings were a ring a perfect ring of mushrooms spaced evenly apart. I read about that in a book years ago.

  12. Neil Scott
    Perth, Australia
    February 21, 2014, 1:25 am

    Scientists come up with a perfectly reasonable explanation for the circles, then some religious person states that it’s “God’s plan”.
    Wouldn’t they be better off proving that there is a God before making such ridiculous and unscientific statements?

  13. Debbie Wathen
    February 20, 2014, 5:05 pm

    They are circles because this is God’s creation and His creation has definite design.

  14. tinker
    February 20, 2014, 3:50 pm

    To the ones that think we are destructive humans ( which we are are ). Quit using elect.gas.propane. etc see if you can

  15. Dena Mistrakis
    Florida, USA
    February 20, 2014, 2:44 pm

    Unbelievable the destruction we don’t see. When are humans going to be held accountable for the destruction they have caused? To do this is be accountable for actions that effect all creatures, big or small.
    Appalled that nothing has been done so far, and expect that when we can no longer eat from these valuable resources it will be to late. Very sad, selfish, warlike humans are. Disgusting !!!

  16. Q
    February 20, 2014, 1:06 pm

    but why always comes in circle???

    Do you know mushroom circles? Starting from one point and spread evenly to all directions. That makes a perfect circle.

  17. Nelson W. Eddy
    East Central Vancouver Island, British Columbia, Canada
    February 20, 2014, 12:43 pm

    The Nile Creek Enhancement Society is working on fundamental support of wild salmon. Cultivating eelgrass, to which herring attach their eggs, is a first step (along with cultivating kelp, which also provides shelter from predators). Go Denmark!

  18. Alvin
    February 9, 2014, 10:30 pm

    but why always comes in circle???

  19. Jaden
    Hubert, NC
    February 9, 2014, 1:25 pm

    I thought fairy rings consisted of mushrooms…

  20. samia
    February 9, 2014, 8:38 am

    I like the national geographic

  21. Atnafu Getamesay
    February 9, 2014, 4:55 am

    it’s so nice