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A vast mass of the seaweed is heading for the beaches of the Gulf of Mexico, probably just in time to wash up and start rotting during the summer holidays. 

It's known as the Great Atlantic Sargassum Belt - Sargassum being the type of seaweed - and it's not new but it is getting much bigger.  In fact, last year scientists estimated its mass to be about 24 million tonnes, large enough to be seen from space.  Climate change and human activity are all thought to be to blame. 

Dr Brian Barnes is a marine scientist at the University of South Florida. Newshour's James Menendez asked him how has this seaweed got so big?

Photo shows: Waters near a Tulum resort are brown from sargassum, a seaweed-like algae, on June 15, 2019 in Tulum, Mexico. Credit: Getty Images