The mystery has finally been solved. The strange mystifying underwater rings was not the work of aliens or fairies after all. It was the result of poison, biologists said.
"Striking rings of green eelgrass - some of them up to 49 feet (15 meters) wide - can occasionally be spotted in the clear Baltic water off the coast of Denmark's island of Møn," NBC News reported. "The formations were captured in tourist photos in 2008 and again in 2011, sparking the type of speculation that's usually reserved for crop circles."
The circles, however, have "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 explained.
"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," Holmer and Borum said in a statement Thursday.
"The biologists concluded that the rings formed because of the radiating pattern in which the eelgrass grows - and dies when exposed to toxins," NBC News reported.
The scientists detected high levels of sulfide, a substance that's poisonous to eelgrass and can build up naturally in a chalky seabed like the one off Møn (or unnaturally when agricultural pollutants enter an ecosystem), in the mud around the eelgrass.
"Most mud gets washed away from the barren, chalky seabed, but like trees trap soil on an exposed hillside, eelgrass plants trap the mud," Holmer and Borum explained. "And therefore there will be a high concentration of sulfide-rich mud among the eelgrass plants."
Though it might resemble a type of seaweed, eelgrass is actually a flowering plant. Circle-shaped colonies are created when it grows and expands outward in all directions.
Although old plants at the heart of the colonies drop dead, healthy adult eelgrass plants seem to be able to withstand the sulfide in their environment, the researchers said.
"The result is an exceptional circular shape, where only the rim of the circle survives - like fairy rings in a lawn," Holmer and Borum added.
Scientists have long been puzzled about other fairy circles on land. "A famous example can be found in the desert grasslands of Namibia in southern Africa, where researchers have offered up a wide range of explanations for the vast field of circular patches, from ants and termites to gas seeps and resource competition," NBC News reported.
The explanation for the eelgrass fair rings is detailed in this month's edition of the journal Marine Biology.