Secrets of the Deep: Blue Planet 2

The second instalment of the eight-part series, The Deep takes viewers on an epic 50-minute journey beyond the twilight zone – the middle layer of the world’s oceans – and into the midnight zone, a place of perpetual darkness, crushing pressure and merciless chill two-thirds of a mile below the surface.

It is Earth’s last frontier; the sea covers 71% of the Earth’s surface, yet less than 5% of it has been explored. “We know more about the surface of Mars than we do about the deepest parts of our seas,” says Sir David Attenborough, 91, who presents the series. “There’s life here, but not as we know it.”

Brine lakes date to the Jurassic era, formed from thick sub-surface salt deposits and dissolved methane. So intensely saline is their content – between five and eight times that of the surrounding seawater — that they cannot mingle with the ocean.

Their margins teem with life — mussels that can live for a century, shrimp and crab feeding nonchalantly. But within their briny depths is the detritus of death — embalmed corpses of those that got too close.

As the team filmed from the submersible, they were stunned to see a cut-throat eel wriggle from the shadows and plunge into the brine lake, as if being swallowed into a black hole. Seconds later, it shoots back out like a torpedo, its body twisting and turning as it goes into toxic shock, contorted by spasms so extreme that it ties itself in knots in a grotesque spectacle.

“It’s total silence, everyone just concentrating on what they’re seeing on the monitors from the sub’s camera. But when it’s over, it’s ‘Oh my God, what did I just see there?’” says Cpt Taylor, 46, speaking aboard the MV Alucia, a 56-metre (183ft) research superyacht that was Blue Planet 2’s floating expedition base.

The ship, to which this newspaper was granted exclusive access during a stopover in Fort Lauderdale, Florida, is owned by American hedge fund billionaire and philanthropist Ray Dalio. It is one of only three ships in the world equipped with two submarines – Nadir and Deep Rover, state-of-the-art vessels fitted with powerful lighting and sophisticated high-definition cameras. Its mission is to pursue and promote greater understanding of the ocean through scientific research and visual storytelling.

Within 48 hours of filming the brine lake, the rubber hoses on the submersible had corroded from the salt, necessitating a total overhaul and replacement. Before filming voracious humboldt squid 800 metres (2,624ft) down in the Pacific, the crew had to bind the sub’s exterior ductwork with tape.

“That type of squid, they are so ferocious. We had to protect all the exposed cables and hoses – if they wanted to, they could rip that right off,” said Cpt Taylor.

Filmed off the coast of Chile, the squid sequence is an unprecedented expose of the ruthless aggression with which the species hunts, capturing on camera for the very first time several hundred of them chasing down bioluminescent lantern fish en masse.

At one point, a squid lunges at the sub’s camera, enveloping its lens with its tentacles loaded with powerful suckers. Another sequence shows the creatures sparring over prey, one of them ultimately turning its attentions instead to seizing a more substantial supper – a rival squid – in a surprise act of cannibalism.

“We’ve seen things that have astonished us. This was one,” said James Honeyborne, the executive producer. “This is a portrait of the deep like we’ve never seen before.”

Other sights included six-gilled sharks feasting on of flesh from a 30-tonne whale carcass – a meal that will set them up for as long as a year before they need to eat again, and “zombie worms” that tunnel into the whale’s bones by releasing acid, in order to feed on the last of the nutrients.

There is a fish – the Pacific barreleye — that has a transparent head to give itself a clear view of predators that may be lurking above, and a cockeyed squid, with one eye that squints upwards and one that peers downwards to keep a lookout. Fangtooth – the midnight zone’s most savage fish – has pressure sensors all over its body to detect movement around it.

Orla Doherty, producer on The Deep, spent more than 500 hours on six submarine dives for the episode, a highlight of which was a voyage to a 50-metre diameter mud volcano at the bottom of the Gulf of Mexico, where all of a sudden giant bubbles of methane the size of basketballs began erupting around her and the crew, trailing sediment behind them like a space rocket trails fire and smoke.

“It’s like being in a sci-fi movie. You’re immersed into this crazy world, like being sat on the surface of another planet – it was so dramatic, we christened it the War of the Worlds. We were sitting there with our mouths on the floor,” she recalls.

One of the key features of the submersibles is their domed cockpits made of acrylic seven inches thick — a departure from traditional portholes, lending greater visibility. “It’s like being in a giant, see-through Fabergé egg,” said Cpt Taylor. “You can be in this eight-tonne vehicle filming a crab the size of your fingernail and still see the individual hairs on its legs.”

As the sub descends, it literally shrinks under the extremities of pressure, which at 1,000 metres (3,280ft) is akin to having eight jumbo jets piled on top of it. Inside the dome, one hears the creaking and groaning of the structure. “It talks to you and reminds you you’re going deeper,” says Cpt Taylor.

Blue Planet also pulled off the first ever manned dive in Antarctica to reach 1,000m depth. On Antarctic dives, icebergs ranging from the size of a small car to the size of Hyde Park collided above them, sending shockwaves through the water. “You feel it more than you hear it. It actually shakes the sub. . . it sends this huge shudder through the vehicle,” says Cpt Taylor.

The Alucia has no home port, crisscrossing the world’s oceans on a constant mission to reveal secrets of the deep. Scientific sensors on its hull are switched on permanently so that wherever it goes, it is gathering and relaying data to the Woods Hole Oceanographic Institution in Massachusetts, such as sea surface temperatures and the amount of dissolved oxygen in the water.

The crew’s experience at sea through the years – Cpt Taylor, for example, spent 14 years in the British Royal Navy as a mine clearance diver before moving into research and exploration – has brought them to the stark realisation that environmental catastrophe is unfolding.

In Antarctica, warming sea temperatures are bringing some usually deep-dwelling species, such as king crabs, into shallower waters. “They’d never been seen 2,000 metres but we witnessed them shallower than 1,000 metres – that’s worrying,” says Cpt Taylor.

Elsewhere, he says, “it’s sad going down to 1,000 metres and knowing you’re the first person to ever eyeball that spot and then all of a sudden you see plastic bottles, tin cans . . . One of the saddest places was the Gulf of Mexico because we saw all the amazing things with brine pools and methane vents but we also saw the destruction from oil leaks and damage to deep sea corals.”

Vince Pieribone, head scientist of the Alucia, said: “The way the ocean’s being destroyed, it’s breathtaking and terrible . . . films like this more than anything help us bring that message to the audience. If people don’t get that soon, it really is going to be hard . . . It’s what drives us, to make this unknown visible.”

Coming 16 years after the first Blue Planet series, Blue Planet 2 aims to galvanise new generations.

Mr Pieribone said: “Knowing that out there there’s a child or a young person watching . . . that’s the next generation inspired. The number of people around the world that say to me ‘I’m now a marine biologist because of the Blue Planet’ – that puts real fire in your belly to get this right.”