REFILE: Stinky seaweed threatens Mexico's pristine beaches, and its tourism An unwelcome visitor has invaded the Caribbean paradise of Mexico's Tulum: Sargassum, a thick seaweed that has turned the crystalline water brown, filled the air with a rotten-egg smell and washed up on the beach in enormous piles that completely cover the white sand -- scientists warn that if no action is taken soon, it could become an "ecological and economic disaster". IMAGES AND SOUNDBITES