Amazon, Borneo, Congo, Daintree. We know the names of many of the world's largest or most famous rainforests. And many of us know about the world's largest span of forests, the boreal forests stretching from Russia to Canada.

Hidden underwater are huge kelp and seaweed forests, stretching much further than we previously realized.

Few are even named. But their lush canopies are home to huge numbers of marine species.

Off the coastline of southern Africa lies the Great African Seaforest, while Australia boasts the Great Southern Reef around its southern reaches. There are many more vast but unnamed underwater forests all over the world. New research has discovered just how extensive and productive they are. The world's ocean forests, we found, cover an area twice the size of India.

These seaweed forests face threats from marine heatwaves and climate change. But they may also hold part of the answer, with their ability to grow quickly and sequester carbon. Underwater forests are formed by seaweeds, which are types of algae. Like other plants, seaweeds grow by capturing the Sun's energy and carbon dioxide through photosynthesis.