Vacationers visit Mexican coastal towns like Cancun, Playa del Carmen and Tulum in search of sun, white sands and clear blue waters. But, recently, they have also experienced something unexpected: sargassum. Sargassum is a kind of sea plant called “seaweed.” It smells bad and is generally unattractive. Now, this seaweed is invading an area of Mexico’s Caribbean coast called the Riviera Maya. It is amassing along beaches and turning the beautiful waters brown. Experts warn that this may be the new normal. Half of Mexico’s tourism profits come from the Riviera Maya. Very little sargassum reached it before 2014. But experts say a possible combination of climate change, pollution from fertilizers and changes in ocean currents has caused the problem to worsen.