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What is it like to stand on the ocean floor? What do you learn from a lifetime spent in the heart of the sea?
Pioneering oceanographers Dr Sylvia Earle and Dr Samantha (Mandy) Joye explain what it's like to forge a career below the waves.
From advising on the filming of the BBC's Blue Planet II to documenting the world's most devastating oil spills, Sylvia and Mandy are two scientists who have seen a lot of ocean.
Between them, they've logged thousands of hours underwater, broken dive records and amassed awards and medals from scientific institutions the world over. Over the course of their separate careers, they've researched everything including microbes, ocean geochemistry, sediments, submarine design and oil spills.
But despite devoting lifetimes to studying the deep ocean, they've still explored just a fraction of what's out there. The deep sea remains largely a mystery to scientists, yet it could be the only place that will protect the future of life on Earth.
Dr Sylvia Earle has been exploring the sea for more than four decades. She was one of the USA's first female aquanauts and has been a National Geographic explorer-in-residence since 1998.
She was also the first female chief scientist of the US National Oceanic and Atmospheric Administration.
In a small room behind the Museum's public galleries, she explained why she's committed to telling the world why we should all get out there and experience the weird and wonderful life forms that exist in the deep sea.
She said, 'To be in the deep sea is to be witness to the greatest spectacle of life on Earth. We are still scratching the surface of the deep sea at about 1,000 metres deep, but there is life beyond that.
'I live for the day that people appreciate that most of the life on this planet lives in the dark all the time.
'The ocean is full of life, and all of it matters, all of the time. We know that the natural world is all tied together, and we are part of it. The more we discover about life on Earth, the more we'll realise our lives depend on it.
'The ocean is truly the heart of the planet but we don't know about it. We are studying how trees produce oxygen, but what about chemosynthesis in the dark ocean? Glimpsing what we don't know about should inspire us.'
Sylvia has spent about 7,000 hours underwater. She knows better than most what the weight of an ocean above you feels like.
When asked whether she feels more at home in the water than she does on the land, Sylvia simply answered, 'I feel at home on planet Earth. Just don't ever send me to Mars.'
She now leads a project called Mission Blue, which aims to 'inspire an upwelling of public awareness, access and support for a worldwide network of marine protected areas,' called hope spots.
Hope spots are defined as parts of the ocean that either need protecting or need further support. They could have an abundance or diversity of species, populations of rare or endemic species, the presence of natural processes such as spawning grounds, or economic importance.
With her vast knowledge about the sea floor, Sylvia was also a consultant on the BBC's recent Blue Planet II series.
She hopes that programmes with such widespread appeal will bring the deep ocean into the public consciousness.
She said, 'I love all of the series, but I am particularly passionate about the deep sea because so little has been uncovered about that part of the planet.'
Working alongside her was Dr Mandy Joye, an oceanographer with more than 25 years of experience in marine sciences. She was the expedition scientist on Blue Planet II's The Deep episode and went with the film crew on many of the dives.
One of the featured sequences was in a deep-sea brine pool in the Gulf of Mexico. This 15-metre-deep salt lake is caused by methane, given off by rotting matter on the seabed. The methane eruptions release brine, which pools together because it is five times heavier than seawater.
Mandy said, 'Brine pools are windows into the past and they are underexplored. They are my favourite places to explore, but we also considered many other sites for the episode, including deep water corals reefs and mud volcanoes. Each place is so different.
'I thought I was pretty good at video underwater, but watching the Blue Planet team was incredible. The cinematography in all the episodes is phenomenal.'
When so much of the open ocean is a great unknown, how did the team know where to dive to get those perfect shots?
Mandy said, 'The hardest thing to contend with during the planning phase was the 1,000-metre depth horizon. The depth limit for the submarines we were using is 1,000 metres down, but there are loads of really interesting sites just below that.
'Finding the perfect balance between finding the stories and working with the technology available was hard.'
Oceanography isn't just about exploring new frontiers. Part of Mandy's work also means she is on the frontline of disaster reaction and prevention.
Her extensive knowledge of the Gulf of Mexico stems from diving and researching there for more than 22 years.
As Professor of Marine Sciences at the University of Georgia, she is studying the impact of the catastrophic Deepwater Horizon oil rig explosion of 2010. The spill from an offshore drilling rig caused millions of gallons of oil to pour into the ocean just off the coast of Louisiana. It killed 11 people and countless marine animals, including fish, birds, crustaceans and mammals.
Mandy went on a dive to the seafloor in the Gulf less than a year after the spill happened, and she said that what she saw there broke her heart.
She said, 'The Gulf of Mexico isn't just a body of water - it is a personal friend to me. It has taught me so much and enriched my life in so many ways, so the spill felt like a personal loss for me.
'It is hard to articulate the impact. I cried when I went on that first dive down to the sea floor. It was brown, covered in a paste of oil.
'I realised there was nothing underneath the paste, nothing on the bottom of the sea. No fish and no shrimps. It was just broken.
'We travelled in a circle, following a two-nautical-mile offset in a submarine and saw nothing alive apart from a single shrimp. That's heartbreaking.'
Seven years on, there has been some ecosystem recovery in the area, but it is slow. Oil residue remains in sediments on the seafloor, and Mandy says that true recovery of the most impacted parts of the deep ocean system could still be decades away, perhaps even 100 years for the damaged corals.
She said, 'I don't think things have really got back to baseline. Other oil spills in the sea have been nearer to the shore, where the pace of change is very different to the deep ocean, much faster.
'We have a long-term dataset and I can see some parts of the area are edging back to where they were before, but it's going to be another 20 to 30 years before the system is recovered.
'For instance, deep water corals grow so slowly, just one metre in 100 years. So for them, recovering from the devastation will take hundreds of years, if they ever recover at all.
'It won't be long until the animals go back to doing what they were doing before the oil spill, but the true restoration of the entire ecosystem will take far longer.'
Sylvia and Mandy were both at the Museum for the Annual Science Lecture 2017, which discussed marine conservation and how the oceans can be protected.
Despite the threats stacked against marine wildlife, including global warming, acidification, plastic pollution, oil spills and commercial fishing, both Sylvia and Mandy believe there is cause for hope in the future.
Sylvia said, 'We have some real thinking to do about extracting wildlife from the sea on a large scale. And we are armed with knowledge. We should pause to think about it.
'We're one of the few creatures on Earth who gather knowledge and pass it on so that each generation has a better idea than the preceding one of what our future is about.
'The good news is that we are beginning to look at things differently. We are experiencing a new wave of understanding of why the ocean matters. There's evidence that when you stop the killing, good things happen. But we have to hurry, and give ocean creatures safe havens.'