As can be seen in the photo, the circles are made up of a dark green, almost black color. These are rings of eelgrass, a flowering underwater plant. Normally these plants create meadows, but here off Denmark's island of Møn, the ocean bottom is chalky, and mostly devoid of mud that the plants like. So they grow by sending shoots outward, and expand radially. These clumps of vegetation accumulate mud, which has high levels of sulfide, as described in a study published this month in the journal Marine Biology. Sulfide is toxic to eelgrass in certain concentrations, and eventually it builds up in the center of these circles, killing the older plants. The younger but well-established plants at the outer rim of the circles survive, however, leaving behind the rings you see in the photo.