Sargassum is a brown seaweed with large, leaf-like structures and gas-filled "berries," which allow it to stay afloat. It is, unsurprisingly, a defining feature of the Sargasso Sea in the northern Atlantic ocean. Sargassum mats are important habit for fish, birds, turtles and crustaceans. A lot of prized fish we love to eat—including mahi mahi and amberjack—use these tangled marine vines as a place to raise their young. "In open waters, it's a good thing," says Wang. "But when it washes onto the coast, it becomes a coastal nuisance." Or worse.