In this episode, Justin Rowlatt meets John Auckland. He is the man behind Seafields, which aims to create a floating farm 'the size of Croatia' far out in the South Atlantic ocean.

The plan is to harvest the seaweed, sink it to the seabed and earn cash from carbon credits.

Justin also speaks to Professor Victor Smetacek, an expert in marine biology - the project is based on his ideas.

And Dr Nem Vaughan, associate professor in climate change at the University of East Anglia talks Justin through some of her questions around how or whether the project will work.

Presenter: Justin Rowlatt

Producer: David Reid

(Image: Sargassum being harvested. Credit: BBC)