Sir David Attenborough reveals the findings of one of the most ambitious scientific studies of our time - an investigation into what is happening to our oceans. He looks at whether it is too late to save their remarkable biodiversity.
Horizon travels from the cold waters of the North Atlantic to the tropical waters of the Great Barrier Reef to meet the scientists who are transforming our understanding of this unique habitat. Attenborough explores some of the ways in which we are affecting marine life - from over-fishing to the acidification of sea water.
The film also uncovers the disturbing story of how shipping noise is deafening whales and dolphins, affecting their survival in the future.