Mysterious circular formations located off the coast of Denmark aren't the work of aliens so much as eelgrass plants growing amid highly toxic mud, according to a team of Danish researchers.

When pictures of the phenomena first surfaced in 2008, speculations of everything from bombs to extraterrestrial life quickly followed. Subsequent studies revealed the formations were made up of eelgrass, but even then mystery remained as eelgrass usually grows as continuous meadows.

"It has nothing to do with either bomb craters or landing marks for aliens. 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," biologists Marianne Holmer from University of Southern Denmark and Jens Borum from University of Copenhagen said in a joint statement.

"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," the two explained.

The substance is sulfide, which accumulates in the seabed where the circles are located because it lacks iron and contains calcium carbonate.

"Most mud gets washed away from the barren, chalky seabed, but like trees trap soil on an exposed hillside, eelgrass plants trap the mud. And therefore there will be a high concentrations of sulfide-rich mud among the eelgrass plants," they said.

The poison is just strong enough to weaken both old and new plants while posing no harm to adult and strong plants. What's more, eelgrass spreads from the inside out, meaning the oldest and weakest plants are located in the center

"Eelgrass populations grow vegetatively by stolons which spread radially in all directions and therefore each plant creates a circular growth pattern," the researchers explained. "When the sulfide begins to work, it starts with the oldest and thus the inner part of the population because here is an increased release of toxic sulfide and uptake by plants due to accumulation of mud. The result is an exceptional circular shape, where only the rim of the circle survives - like fairy rings in a lawn."

The problem of sulfide poisoning is much wider than these particular rings, Borum and Holmer note. The substance frequently appears in those places where oxygen is lacking, which can happen when an area is inundated with nutrients derived from agriculture.

This is a problem, they explain, as underwater meadows of eelgrass and other types of seagrasses represent home for many small animals in addition to filtering the water and trapping both carbon and nutrients - a fact that has prompted scientists and authorities in places like Denmark to work together to protect seagrasses before it's too late.