David Attenborough talks about the making of Blue Planet II, and the peril of plastic in our oceans

For Blue Planet II, the sequel to the landmark 2001 BBC series, producer Orla Doherty and her team spent a total of 500 hours at depths of 1,000 metres in Earth’s “inner space”. And on her first Antarctic dive, descending into frigid waters of minus 1.6 degrees Celsius, things didn’t quite go to plan. At 450m, she noticed a puddle “gathering at the bottom of the sub”.

Sir David Attenborough, who at 91 is no longer able to join such voyages in person, watched the footage jealously from the safety of a studio while he put together his narration. “I would have thought what that water was if it had been me!” he jokes.

There are no toilets on the submersible – the least of the problems faced by its inhabitants during underwater stints of up to 10 hours at a time. “But it wasn’t! It was seawater,” he confirms with a chuckle (the only way of determining this, apparently, is to dip your finger in the liquid and taste it).

Suffice to say, Doherty survived to tell the tale (an adventure that included spotting fish with antifreeze in their veins and swarms of krill that glow in the dark).

It is just one example of the lengths and depths to which the naturalists have gone to explore beneath the waves. They custom-built a “Mega-dome” camera so they could film above and below the waterline at the same time. They were the first professional filmmakers to document false killer whales and bottlenose dolphins – two entirely different species – socialising together; octopuses wearing shells and rocks as body armour; and giant trevally launching themselves out of the sea to prey on fledgling terns in mid-air. Twelve scientific papers are already in the pipeline off the back of the behaviours and habitats uncovered.

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If the first episode is anything to go by, Blue Planet II is a banquet for the eyes and a fiesta for the ears, with a soaring score from Oscar-winning composer Hans Zimmer. It boasts high drama, comedy, romance and pathos.

Moreover, Asia takes a starring role, with Asian sheepshead wrasse filmed in Japan, broadclub cuttlefish shot in Indonesia and coral reefs captured in the Philippines.

Amid the enchantment, the producers have not shied away from the horror – laying bare the chaos we have wreaked on our oceans.

“When I was at school,” Attenborough recalls, “I remember very well the science master coming and saying: ‘Boys, we used to live in the steam age, and then we lived in the electricity age, but now, the age you’ll be living in is the age of plastics. Scientists have gone to a lot of trouble to produce plastics that are indestructible! Isn’t that wonderful!’

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“And, of course, what we do now is throw thousands of tons – a day – into the sea, where it doesn’t rot. Worse than that, it breaks up into little fragments. Birds will travel round the world collecting squid or fish, come back after three weeks to feed their nestlings – and we filmed them – out of the beak of the mother comes, not food, bits of broken plastic.”

This is “not an axe-grinding series”, he is at pains to proclaim. But, arguably, the messages it conveys could not come at a more crucial time, as the most powerful man in the world pulls the biggest polluter per capita out of the UN Paris climate change agreement.

If US President Donald Trump were in front of you, I ask, what message would you want to deliver? The broadcaster shoots me a look a pyjama shark might give a common octopus, before adding sharply: “Well, the obvious message of what the films say: ‘Just please look at the films, see the evidence’.”

Attenborough is slightly more forthcoming when he references the subject a little later.

“I get between 20 to 30 letters a day. The overriding thing that people are saying is that when you’ve got all the worries in the world that there are, with Brexit, with North Korea, with Trump, just to be able to look at something in which human beings aren’t the prime player, where there is beauty, wonder, astonishment, history, biology, and above all, truth [he fixes me with a stare as he emphasises the word], which is not as common a commodity on television as you might think …” Attenborough pauses before adding with a smile: “We come home at night – and that’s what we want to watch.”

Blue Planet II premieres on BBC Earth (Now TV On Demand) at 4am on October 30. The episode will be shown again at 8pm on October 31.