As he nears the end of his life, there's one thing David Attenborough wants you to know.

Sir David Attenborough has achieved a lot in his 99 years, and as he launches his new documentary, he's sharing his most urgent lesson.

His new cinema-length film Ocean with David Attenborough was released in cinemas today — the same day the legendary British broadcaster, historian and filmmaker celebrates his milestone 99th birthday.

Attenborough addressed his mortality in the documentary and admitted he's "nearing the end of his life."

"After living for nearly 100 years on this planet, I now understand that the most important place on Earth is not on land, but at sea," he said.

"To this day, we have seen more of other planets than we have of the ocean. Now we are making discoveries that completely change our understanding and could offer a better future for everyone on Earth forever."

Watch: The official trailer for David Attenborough's new documentary. Post continues below.

A page from his new book, which was shared in The Sunday Times, detailed a heartfelt tribute to nature, which he fell in love with as a young boy while exploring the old limestone quarry near his childhood home in Leicester.

He confessed that the ocean's recovery and restoration is one thing he won't live to see.

"We have discovered more about our ocean than in any other span of human history," he told the newspaper.

"I will not see how that story ends but, after a lifetime of exploring our planet, I remain convinced that the more people enjoy and understand the natural world, the greater our hope of saving both it and ourselves becomes."

He added that the next generation will see the consequences of human action, or inaction.

David Attenborough and King Charles III at the Ocean With David Attenborough world premiere in London, England. Image: Getty.

"Young children playing on a beach today will live through perhaps the most consequential time for the human species in the past 10,000 years," he said.

"They will grow up to see how this story ends, to see how our choices play out."

Attenborough is the longest-serving naturalist and TV presenter in history, having begun his career with the BBC in 1952.

His filmography as a writer, presenter and narrator spans approximately 70 years and includes landmark series such as Natural World, Wildlife on One — BBC's flagship nature documentary series — the Planet Earth franchise, The Blue Planet and its sequel.

What we know about David Attenborough's new film.

Attenborough's latest documentary serves as a plea to conserve the world's oceans before it is too late.

It showcases threats such as over-fishing, plastic pollution and climate change, and urges countries to create fully protected marine reserves in 30 per cent of their waters.

Ocean with David Attenborough is released in cinemas today. It will hit streaming services in June. Image: Keith Scholey/Silverback Films.

In one powerful sequence, cameras follow a bottom trawl, where nets are dragged with a metal beam across the seabed to catch fish. As it travels over the ocean floor, it destroys the habitat, stirs up silt and scoops up species.

The film also documents the damage climate change is doing to coral reefs as the world's oceans warm, causing mass bleaching events and harmful algae growth.

Yet amid the sobering realities, Attenborough delivers what may be his most important message — one of hope and action.

"The ocean can recover faster than we thought possible," he said. "If we just let nature take its course, the sea will save itself."

"If we save the sea, we save our world," he added.

The film will be released on National Geographic, Disney+ and Hulu on June 8th, coinciding with World Oceans Day, June's United Nations Ocean Conference 2025 in Nice, France, and midway through the United Nations Decade of Ocean Science for Sustainable Development (2021-2030).

Feature image: Keith Scholey/ Silverback Films.