Since the first images of the mysterious ocean circles off the Baltic coast of Denmark were taken in 2008, people have tried to find an explanation, researchers at the University of Southern Denmark and the University of Copenhagen said.
"It has nothing to do with either bomb craters or landing marks for aliens," biologists Marianne Holmer and Jens Borrum said. "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."
The circles consisted of eelgrass plants growing on the bottom of the shallow water, they said, but the puzzle was why they were growing in circles, since eelgrass usually grows as continuous meadows on the seabed.
"We have studied the mud that accumulates among the eelgrass plants and we can see that the mud contains a substance that is toxic to eelgrass," Holmer and Borum said.
The poison is sulfide, a substance that accumulates in the seabed, they said.
Sulfide is toxic enough to weaken both old and new eelgrass plants but not toxic enough to harm strong, adult plants, and since eelgrass spreads radially from the inside out the oldest and weakest plants are located in the center of the growth circle, they said.
"The result is an exceptional circular shape, where only the rim of the circle survives -- like fairy rings in a lawn," the researchers said.