The third instalment took nearly four years to make and comes around 25 years since the first series premiered

Sir David Attenborough has promised to "venture even further" and uncover marine life that is "more precious than we ever realised" in Blue Planet III.

The broadcaster, who turned 100 earlier this year, narrates the new six-part series exploring the darkest depths of the world's oceans, produced by the BBC Natural History Unit in Bristol.

After almost four years of filming, the latest instalment in the franchise will air on the BBC later this autumn, marking 25 years since the first series premiered.

At a launch event on Tuesday, Sir David said: "When I narrated the first Blue Planet series, it didn't occur to me that I would be narrating a third series in my centennial year."

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The production uses advanced equipment including remote underwater cameras, drones, and manned submersibles to capture newly discovered animal behaviours.

Viewers are taken into some of the world's least understood marine environments, ranging from coral reefs and tropical islands to the darkest depths of the ocean.

In the newly released extended trailer, Attenborough promised to "venture even further" than ever before, describing the ocean as "an endlessly fascinating place".

"We will uncover life that is more surprising, more intelligent, and even more precious than we ever realised," he said.

"We will discover the extraordinary relationship we have with the ocean. One that has never been more important to understand."

Sir David said he was "proud and delighted" to be associated with the project.

Sir David said it "has never been more important to understand" the ocean

The new series is accompanied by an original score from Hans Zimmer and Bleeding Fingers Music.

Zimmer, who also scored Blue Planet II, said the series was the "most important work" he has ever done.

"I think everybody who comes to Blue Planet comes to it with the knowledge that it is something that changes the world by making you fall in love," he said.

"It's not something where you're being lectured or cajoled about things, but it's something that just shows you how spectacular and beautiful our world truly is.

"How we need to preserve it and how we need to love it, and how whatever we love, we need to treat with great kindness and great respect."

The series examines the habitats and behaviours of animals including humpback whales, dolphins, and bioluminescent deep-sea creatures

It also features an original song, Into The Deep (Seas Cry) co-written by Jade Thirlwall - best known for being a member of the pop group Little Mix.

Thirlwall described herself as a "huge fan" of the franchise, praising Attenborough as "a true hero".

"Being part of Blue Planet III in this way feels incredibly special and a real privilege," she added.

The premiere launch event took place at Piccadilly Circus in London on Tuesday, with footage from the new series taking over the digital displays.

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