sciencehabit writes "The truth behind the mysterious underwater circles that periodically appear off the coast of Denmark has been discovered, and sadly it doesn't involve aliens, fairies, or the fabled lost city of Atlantis. In 2008, a tourist snapped photos of several large dark rings that appeared near the white cliffs of Denmark's island of Møn in the Baltic Sea. The circles, several as large as a tennis courts, sparked numerous theories of their origin—some more outlandish than others. In 2011, when the formations reappeared, scientists discovered they were actually round bands of marine eelgrass, similar to rings of mushrooms known as fairy rings. Because eelgrass usually grows as continuous underwater meadows, scientists were still baffled by the rims of lush eelgrass with barren cores. Now, researchers say they at last know the rings' true cause."