Record amounts of sargassum are floating in the Caribbean Sea and ending up on beaches from Puerto Rico to Guyana — but scientists aren't sure why there's so much of it in the first place.

A record bloom of sargassum in places like Puerto Rico and Mexico is impacting beachgoers with its strong smell and can cause skin irritation for swimmers. But as NBC News’ Chase Cain explains, the harm it causes to the environment goes beyond just a bad scent.