Scientists have ruined a perfectly good mystery off the coast of Denmark—weird circles spotted on the ocean floor aren't so magical after all, despite their nickname of "fairy rings." Nor are they caused by World War II bombs or aliens, two of the more interesting theories. No, as Nature World News explains, the rings are actually caused by "toxic mud," or at least mud that's toxic to eelgrass.The Danish researchers discovered that the seabed near the circles has high levels of sulfide, possibly because of runoff from local farms.
That sulfide would normally get washed deeper out to sea, but it gets trapped by the eelgrass near the coast, much the way "trees trap soil on an exposed hillside," say the researchers, quoted in LiveScience. The last piece of the puzzle: Eelgrass tends to grow in circular patterns, with the older, weaker plants in the center. Those are the ones that tend to die because of the sulfide, and "the result is an exceptional circular shape, where only the rim of the circle survives—like fairy rings in a lawn," say the researchers. (The lawn rings are usually explained by fungi, notes LiveScience.) The mystery may be explained, but the scientists say it raises the importance of protecting seagrass in general, a key part of marine ecology.