The mystery of the underwater circles in the Baltic Sea off the coast of the southern Denmark island of Møn has finally been solved, and it turns out that the ring-shaped markings are not the work of man, aliens or God, but nature. 

The mystery had caught the attention of outlets across the world, from the Los Angeles Times to the Daily Mail, although to be fair to most of the Danish media, they didn't show a huge interest in the new findings.

Eelgrass, really

It all began in 2008 when a tourist spotted strange rings beneath Møns Klint, the famous white chalk cliffs of Møn Island. The circles were as large as 15 metres in diameter.

In 2011 the rings were seen again, and on this occasion, scientists reached the conclusion that the shapes were made of eelgrass. Still, it has taken them until now to be able to explain why the grass grows in circles.

A toxin is to blame

“We have investigated the mud, which is gathering within the eelgrass circle, and have detected a substance that is toxic to the eelgrass,” explained biologists Marianne Holmer from the University of Southern Denmark and Jens Borum from the University of Copenhagen in a press release.

The substance is called sulfide and it kills the grass in the centre of the rings where the mud is build up.  Along the perimeter the layer of mud is thinner and it therefore allows the eelgrass to grow. So there you have it: mystery solved.