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Earth may be the blue planet, but beneath the ocean waves lies a vast forest of green.
Often, kelp forests and other algae are the ones that support these watery worlds.
Kathryn Jeffs, producer on the Green Seas episode of the BBC's Blue Planet II series, introduces some of the heroes of the ocean that emerged during filming.
The ocean covers more than 70% of Earth's surface, but there is far more to it than meets the eye.
Kathryn says, 'Everyone thinks about our planet's oceans as blue - it's the blue planet. But actually it is the green seas that bring much or most of the life to our oceans.'
For the producer, one of the most surprising locations featured in the Green Seas episode were the kelp forests along South Africa's coastline.
'It was a completely unknown world to me. They were astounding in their diversity and colour.
'We are so much more familiar with land-based habitats like jungles, but on the tip of South Africa there are these phenomenally rich, beautiful kelp forests that are full of amazing and very intelligent life.'
These forests provide structure and homes for the animals that reside in the ocean. They provide fish with nurseries and hiding spots from predators.
'Many of our commercial fish start their lives in these more structured green seas. Many of the tropical reef fish start their lives in the mangroves or in sea grasses.
'The green seas are at the base of almost all the food chains, of all life in the seas - the algal blooms that feed the food chains, the temperate seas and the great schools that feed the whales.'
When it comes to intelligent creatures, viewers needed look no further than a common octopus caught on camera, exhibiting previously undocumented behaviour.
The little female gathered as many shells from the seafloor as her arms could hold. Surrounding herself with them, she created camouflaged armour that allowed her to hide from predatory pyjama sharks swimming through the kelps.
But the octopus's lesson in the art of escape did not end there. When a shark did attack her, she fought back, pushing her long, suckered arms into the assailant's mouth and out through its gills, halting its breathing until it let go.
'The behaviour came completely as a surprise to the cameraman and Craig Foster, the local naturalist they were working with.
'These things are incredibly rare. It was only by spending so much time in the field with the individual octopus that they got to know very well, that they were there at the exact moment when the attack happened.'
The canny tactics made the cephalopod one of Kathryn's favourites of the episode.
'They're just so alien to us. Their intelligence has really evolved in a separate way, so it should be difficult for us to understand them.
'But when you meet them, they're interesting and sensitive - they want to explore and learn. They're tactile and slightly nervous, which gives them a vulnerable quality. They can be slightly comical and we read that into them, but they're just amazing, clever animals.
'It would be amazing to understand the intelligence of an octopus and how it perceives the world. We could only glimpse a fraction of that through working closely with them.'
Blue Planet II captured the imaginations of millions through its stories of life beneath the waves.
'I think we all relate to a story,' says Kathryn. 'We relate when we can see emotions, struggles, conflicts and triumphs. We like rooting for a hero.
'We have to be careful to never anthropomorphise. We are following a route whereby we are showing absolutely natural behaviours. Yet in natural stories, heroes emerge. Life as an animal - surviving, finding a mate, finding food - really is heroic in of itself.
Kathryn believes that the characters that emerged on screen have an important part to play in awareness and encouraging viewers to understand and protect the oceans.
'It's only once you fall in love with things that you see the value of them in our world and want to take care of them.
'I've never had to try to love animals or have an interest in them - it's really an honour to share that passion with other people. If we instil a love of wildlife and interest in the natural world in children as they grow, they'll take that with them.
'It's such a fascinating world - why not learn about it?'