For a long time, odd underwater spherical structures in the Baltic Sea have mysterious occupants. Some have believed that they were bomb craters, or perhaps made by fairies or aliens. So happens, none of the above answers is right.

“It has absolutely nothing to do with either bomb craters or even landing imprints for aliens,” claimed scientist Marianne Holmer who is from University of Southern Denmark along with Jens Borum from University of Copenhagen in a press release.

As can be found in the picture, the circles are composed of a cloudy green, roughly black color. They are rings of eelgrass, a budding marine plant. Usually these plants produce meadows, but right here off Denmark’s island of Møn, the sea bottom is flaky, and mainly without any mud that the plants want. So that they grow by transmitting shoots to the outside, and expand radially.

These types of patches of vegetation gather muck, which has increased levels of sulfide, as explained in research posted this month . Sulfide is harmful to eelgrass in various levels, and ultimately it accumulates in the core of these rings, eliminating the older vegetation. The newer but well-established vegetation at the outer rim of the rings thrive, nevertheless, leaving the bands you observe in the photograph.