Bringing an underwater city to life

I remember watching the Cities episode of Planet Earth II and being completely captivated by the way the camera appeared to fly effortlessly through a city. One moment we were soaring above skyscrapers, the next we were racing through the underground, all in what appeared to be one continuous shot. I'd never seen a city filmed quite like it.

The technique is called flow motion, and it immediately got me thinking about coral reefs. In many ways, a coral reef is just like a bustling city. There are towering buildings, busy streets, rush-hour traffic and even cleaning and waste removal services. What if we could take audiences on a journey through a coral reef in the same way, making them feel as though they were diving through this extraordinary underwater world themselves?

I was working as a researcher on Blue Planet II at the time, and although we explored the possibility, the technical challenges proved too great. But the idea stayed with me, and when I started work on Blue Planet III, I knew I wanted to give it another go. Our team's ambition was to show how tiny coral polyps build vast, solar-powered underwater cities, and how the extraordinary animals living within them all play their part in keeping these bustling communities alive. My Series Producer, Lara Bickerton was immediately enthusiastic. But getting everyone excited was the easy part. The real challenge was figuring out how on earth we were going to pull it off!

We knew we had to speak to Rob Whitworth, a flow motion expert who had helped create the incredible Cities sequence in Planet Earth II and had since used the technique to spectacular effect in Planet Earth III. Rob was up for the challenge but there was just one catch – he wasn't a diver! In fact, all his previous dedicated flow motion sequences had been filmed on land. So that's where cinematographer Alex Vail came in. He had already experimented with filming flow motion and with a PhD in coral reef ecology, knew the reef and its inhabitants intimately.

What followed was months of planning, countless phone calls and plenty of head-scratching, as we got to grips with Rob's flow motion techniques and Rob discovered the unique complexities of filming underwater. One of the biggest challenges was figuring out how to move a camera smoothly through such a complex underwater environment. We wanted audiences to feel as though they were gliding effortlessly between coral structures, then suddenly racing through shoals of fish. Alex's solution was to develop new underwater camera rigs specifically designed for flow motion.

Alex explains: "I built a large underwater blimp rig, which needed an incredible 40kg of ballast just to achieve neutral buoyancy. This allowed us to capture beautifully smooth, gliding shots through the reef's bustling streets. We also used a smaller camera rig that, combined with leg-mounted underwater scooters, allowed us to race through the reef, capturing the action from a perspective similar to that of an FPV (first-person view) drone."

With the equipment ready and an ambitious shot list in hand, my Assistant Producer, Katy Moorhead, travelled to Lizard Island on Australia's Great Barrier Reef with Alex and cinematographers Bérénice Mathieu and Natalie Turner-Blackman. At the time, Lizard Island was home to spectacular coral formations and thriving communities of reef fish, making it an ideal place to bring our underwater city to life. Over two weeks, the team experimented with new angles and perspectives, filming in a way that was very different from a conventional natural history shoot.

Katy: "This was undoubtedly the most challenging sequence I've ever worked on. It wasn't enough to capture a beautiful shot of a fish or a coral formation. Every movement had to be carefully considered, with the team constantly thinking about how one shot might join seamlessly to the next. Then there were the variables we could never control: the sun, the clouds, the swell, the current, and, of course, the animals themselves. We could plan every camera move in detail on land, but the natural world rarely follows a script. Yet that unpredictability was also what made it so rewarding. When every element aligned, even briefly, the results were extraordinary."

But our ambition went beyond taking audiences on a journey through the reef. We also wanted to reveal how these incredible underwater cities are built, from the tiny coral polyps responsible for creating structures over 2000km long. Capturing individual polyps, some no bigger than a pinhead, required controlled studio conditions and specialist filming expertise. At the other extreme, drones allowed us to reveal the sheer scale of the structures these tiny animals create.

Once filming was complete, the challenge shifted to the edit. Somehow, all these different shots had to come together as one seamless journey through the reef.

Rob mentioned "What I love about flow motion in natural history is its ability to visually connect the dots. There's a moment where we dive down into a deep ravine before sweeping upwards to reveal these enormous coral towers. It's a wonderful way of taking audiences on a journey through the reef, revealing its extraordinary scale and complexity. The transitions are technically challenging, but the real satisfaction comes when they become invisible and the pictures tell the story."

By using flow motion to introduce the reef, our team was able to bring coral reefs to life in an entirely new and immersive way, taking audiences on an exhilarating journey through an ecosystem thousands of years in the making. And that's particularly important for the story we tell later in the episode, when we witness the devastating effects of an underwater heatwave and coral bleaching. Having experienced the reef as a thriving, interconnected community, I hope audiences will have a greater appreciation of what is at stake. Coral Reefs are more than simply beautiful underwater landscapes. They are living, interconnected ecosystems, built by countless tiny animals, that support an astonishing diversity of life. They play a vital role in the health of our oceans and urgently need our protection.