Baltic Sea ‘Fairy Rings’ Explained by Denmark Biologists

The unusual dark circles that appeared in the shallow waters of the Baltic Sea were first spotted and photographed by tourists in 2008. Since then, these circles (also known as 'Fairy Rings') have been a subject for debate in the geoscience world.

Others who saw the pictures of the dark circles began to come up with farfetched explanations for their existence. Many questioned whether they were left by alien spaceships or if they were remnants from World War II bomb craters. Those accusations can be laid to rest because it appears that Danish biologists have finally determined their origin.

"It has nothing to do with either bomb craters or landing marks for aliens," biologists Marianne Holmer from University of Southern Denmark and Jens Borum from University of Copenhagen said in a statement from this International Business Times article. "Nor with fairies, who in the old days got the blame for similar phenomena on land, the fairy rings in lawns being a well-known example."

These teams of researchers from the University of Southern Denmark and the University of Copenhagen found that a marine plant called 'eelgrass' is the cause of these dark circles. They said the eelgrass grows in large circles in the shallow water. The stronger grass is located on the outside of the circle, whereas the weaker grass is located in the center of the circle.

As for the black coloration and hollowing of the circle, the Baltic Sea contains toxic mud that gets caught in the eelgrass. The eelgrass traps the mud, and the sulfide in the mud begins to kill the eelgrass. When the eelgrass begins to die and wither away, the middle area becomes empty first and leaves a hollow circle (due to the locations of the weaker and stronger grasses). Finally, the trapped mud creates the black color of the hollow circle.

The researchers' study was published in the journal Marine Biology where they further explain the descriptive process of how the circles form.