Streaks of sargassum in turquoise water

Most of the Caribbean's waters move at the whim of the North Atlantic Gyre. This system of currents encircles the calm, clear Sargasso Sea, an expanse named for the enormous quantities of sargassum seaweed found within—though more and more of it is escaping. Rising temperatures combined with nutrient-rich waste runoff have made the sea more prone to algae blooms, which form large masses when smushed together by the gyre. The biggest such mass, the Great Atlantic Sargassum Belt, now stretches 5,000 miles across the ocean. The stinky seaweed is a bane for beachgoers, but seaborne bands of the stuff look pretty cool from the air.