Sargassum seaweed bloom threatens Florida beaches.

The great Atlantic sargassum belt, a 5,000-mile-long mass of buoyant algae, is circling around the Gulf of Mexico and mid-Atlantic, threatening beaches along the Florida Keys, Mexico's Yucatán Peninsula, and the eastern Caribbean. The sargassum mass is expected to cause headaches for beachside towns and tourists, as it can smother coral reefs, alter water's pH balance, and choke local economies. The exact drivers of the growth are still a bit "shrouded in mystery," but researchers believe that excessive sargassum bloom can be mitigated by cleaning up major rivers from the Mississippi to the Orinoco.