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. The scientists found large amounts of toxic sulfide built up in the muds where the eelgrass grows. The sulfide forms when nutrients from agricultural runoff cause bacteria to flourish. Eelgrass grows radially outward, with older plants in the middle and younger seedlings on the outer rim. Because only the middle ring of mature plants can endure the poisonous sulfide, a near-perfect ring of seagrass forms, the researchers report in the February issue of Marine Biology. While the eelgrass circles make for a remarkable sight and a catalyst for kooky conspiracy theories, the researchers say sulfide from agricultural runoff has become a major problem for seagrass ecosystems worldwide.
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