Sun, surf and beautiful sand. Mexico's beaches in Cancun and Tulum have everything a tourist could ask for. And now something they wouldn't: stinky seaweed. In recent years, sargassum has been piling up on beaches and turning waters brown along the Caribbean Coast. And that's valuable real estate. It accounts for nearly half of Mexico's tourist income. Scientists say climate change and intensive use of fertilisers are the reason behind the phenomenon. They're warning it may be the new normal. And that's risking the livelihoods of locals who depend on tourism dollars.