In 2011, scientists interested in mapping seafloor habitats sunk cameras into the chilly waters of a glacial fjord in Norway’s Svalbard archipelago in the Arctic Ocean. Later, when analyzing the photographs, they spotted something unexpected growing in the murky depths: an abundant, dark brown macroalga (seen above) attached to stones and waving in the current. At 166 meters, the seaweed—which needs some light to survive—is the deepest known living macroalga in the high Arctic, the scientists report online this month in Marine Biodiversity Records. Not many fauna seem to find the dark bottom an appealing place to live; a pink sea star and an unknown creature that could be a bryozoan—an aquatic invertebrate also known as a moss animal—were the only other species observed among the white shells of dead clams. Other studies have documented some macroalgae living at greater depths—down to 268 meters—in temperate and tropical climates, but the water there is much clearer, allowing more sunlight to filter through. In polar regions, however, scientists have documented that increased sunlight can actually inhibit photosynthesis in macroalgae that are adapted to survive the long, dark winter nights and ice cover. In the future, the scientists plan to collect specimens of their find to help verify that what they see is in fact a macroalga and, if so, what kind. Such taxonomic identification will be the first step in understanding what adaptations allow the mysterious organisms to make their home on the fjord’s dimly lit sea floor.