David Attenborough reveals terrifying time he almost drowned when testing scuba diving equipment

Sir David Attenborough's first dive to see the Great Barrier Reef sticks with him for a lot of reasons - including the fact that he almost drowned.

The national treasure, who celebrated his 99th birthday last month, said that this underwater expedition in 1957 is the 'moment he remembers most vividly' out of all his travels.

And considering he's visited pretty much every far-flung corner of the world while filming wildlife documentaries, that's saying something.

Nearly seven decades ago, the broadcaster and biologist was on his way to New Guinea when he made a stop off in Australia to dive down to the Great Barrier Reef, even though he was 'no sort of underwater swimmer'.

Attenborough told The Guardian in 2015: "I didn’t know what I was doing! From all my travels, the first time I put on scuba gear and dived on the coral reef is the moment I remember most vividly.

The 99-year-old almost drowned during his first ever dive down to the Great Barrier Reef (Dave Benett/Getty Images)

"Suddenly, this amazing world with a thousand things you didn’t know existed is revealed right in front of you, all wonderful colours and shapes.

"Just the very thing of going underwater itself is exciting, as suddenly you are released from the shackles of gravity and you can move anywhere - great shoals of fish, the reef is as rich as it comes, as varied and as beautiful as it gets."

But what he didn't mention during the interview a decade ago is that he almost drowned while testing a scuba-diving helmet on the dive to the world's largest coral reef system in 1957.

During a discussion with Prince William about his latest documentary, Ocean, Attenborough explained how his headgear ended up flooding with water when he went below the surface.

Take a look at what he had to say here:

While sitting down with the heir to the throne at the Royal Festival Hall in May, Attenborough hilariously tried on an old school-style open-circuit scuba helmet.

He grabbed the equipment which was famed by French oceanographer Jacques-Yves Cousteau, as he told William that wearing one was 'certainly a strange thing to do'.

As he popped the helmet over his head, the Prince of Wales appeared to panic as he told Attenborough while laughing: "Don’t do that, David! You won’t get it off."

After removing the protective face covering, the Blue Planet star began reciting the events of what happened the first time he'd put one on.

"When I put mine on for the first time, I suddenly felt water coming around [my chin and up over my mouth]," Attenborough said. "I thought, ‘This can’t be right’.

"And by the time [the water rose to my nose], I thought, ‘I’m sure this is not right’."

Sir David Attenborough told Prince William he 'couldn't breathe' (YouTube/The Prince and Princess of Wales)

"But then, of course, if you got this thing screwed on top of you, you can’t breathe," Attenborough continued. "You can’t even make yourself heard, you know: ‘Get it off me!’"

The environmentalist then explained that the bloke who was 'directing the operation' thought he was overreacting and didn't really believe that there was a fault.

"So he put it on and I’m happy to say he went under the water and came up even quicker than I did, because there was actually a fault," Attenborough laughed.

The first screening of Ocean With David Attenborough took place at the Southbank Centre in London on 6 May, and the TV star took the opportunity to explain why ocean recovery is so important for our planet.

"After almost 100 years on the planet, I now understand the most important place on Earth is not on land, but at sea," Attenborough said.

Promising to be 'very different' to his other projects, one of the directors of Ocean said: "Nothing has come close to how important this film is."