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Close-up of Sargassum seaweed, showing the air bladders that help it stay afloat, after accumulating on Soliman Bay Beach, Tulum, Quintana Roo state, Mexico on August 17, 2018. Sargassum—a brown seaweed that lives in the open ocean—has overwhelmed shorelines along the East Coast of the United States, Gulf of Mexico, and the Caribbean. Researchers say that the Sargassum outbreak started in 2011, but it has become worse over the years and could cause a serious environmental disaster. As the Sargassum is cleaned up on the shorelines, in a matter of days the shorelines are once again filled. When the Sargassum seaweed lands and starts to decompose, a toxic hydrogen sulfide gas is released, spreading an unpleasant odor and potentially causing adverse health effects. Apart from being an annoyance for tourists, Sargassum also affects oxygen levels in the water, brings contaminants and other microorganisms to the beach, changes the ecological balance of coral and causes beach erosion when its removed in such large quantities. There are several factors that could explain the proliferation of Sargassum in recent years. These include the rise of sea temperatures, the change of sea currents due to climate change and nutrients dumped into the ocean from farming and deforestation around the world. (Photo by Bénédicte Desrus)