Imagine holding your breath for more than four minutes. If that already seems beyond impossible, imagine doing it underwater - deep underwater - descending 100m down into the darkness.

As your heart rate slows, your blood circulation begins to short circuit, cutting off the flow first to your fingers and toes, then your hands and feet, then your arms and legs, leaving more blood for your heart and brain to use.

Then there's the 'blood shift', which starts as soon as you enter the water but increases as you go deeper, pushing blood into the thoracic organs, particularly the lungs. It's deeply uncomfortable, but keeps pressure inside the chest high enough to allow you to proceed deeper without your chest collapsing.

Supplied William Trubridge reaching the marker more than 100m below the ocean surface.

Once you go down as deep as you can go, you then have to come all the way back up again.

This is freediving, and for a rare breed of humans, including New Zealander William Trubridge, it's a sport, a lifestyle and an addiction.

READ MORE:

* William Trubridge sets two freediving world records in three days

* NZ freediver William Trubridge in 'huge shock' at loss of Natalia Molchanova

* Kiwi freediver dices with death in failed record attempt

William Trubridge dived to 125m on a single breath of air in the Bahamas.

On Thursday 21 July at 7:40am NZ time, Trubridge will attempt to freedive to 102m, completely unassisted, in a single breath, into the deepest blue hole on earth. If he succeeds, it will be the furthest underwater any human has ever been without the assistance of breathing apparatus, ropes or even flippers.

He's already attempted it once, in December 2014, but was forced to pull on his safety rope in the last 10m of his descent.

SUPPLIED William Trubridge.

"On that dive I realised during the ascent that I was going to run out of oxygen and not make it to the surface under my own steam," says the 36-year-old, who currently lives in the Bahamas, where Dean's Blue Hole is. "I made that decision as late as possible, as I really wanted to give it my best shot."

Trubridge knows his limits, a vital characteristic in an extreme activity that claims an estimated 100 lives each year. But he also knows when he can push them, and each new record attempt is just that. Many of the records he breaks are his own.

Unsurprisingly, it's a discipline that requires pretty intense levels of dedication. The 17-time world record-holding freediver and current freediving world champion trains 11 months out of 12, three to six hours a day.

New Zealander William Trubridge sets a free diving world record with a 122m plunge in the Caribbean.

"I complement my training with yoga, some weights and other dry exercises, but it's all targeting areas that help flexibility, power and breath hold. I try and keep it as specific as possible, so any other sports I practice are purely for fun."

Freediving is something most people view with a level of both fascination and fear, but Trubridge says the risks are "actually very manageable."

"When we're diving with competent safety divers there's a lot less that can go wrong than most people would expect. Compared to other so-called 'extreme sports,' freediving is probably one of the safest. The greatest danger is your own decision-making."

And for the more than 5000 freedivers around the world, the benefits, both mental and physical, outweigh any fears they might have. "Freediving is a healthy sport that will develop cardiovascular and pulmonary systems as well as work all muscles evenly," says Trubridge.

"Mentally it develops skills of relaxation, discipline, patience, confidence, autonomy and more. Beyond that I believe it's also healthy for the planet, as it fosters a greater connection with the seas that motivates people to do more to protect and respect them."

Trubridge spends the hours leading up to a dive trying to relax, listening to music and visualising the dive one more time, "with a little stretching for the body and lungs."

"For this dive the difficulty is in not spending time contemplating the various success/failure scenarios, which raise my level of nervousness before the dive. So I just try and stay focused on the present moment and whatever I'm doing in it, whether that's putting on my gear, driving to the Blue Hole or just relaxing and breathing."

So what does it feel like down there, and what will Trubridge be thinking about when he attempts his next record-attempt dive?

"It's quiet and I have my eyes closed. During a dive I'm trying to slow the mind down as much as possible, by focusing on the spaces between thoughts.

The first thing Trubridge will need to do when he makes it back to the surface? "Breathe! Actually it does take a while to stop panting after a deep dive, as my muscles are trained to work anaerobically, so there is a lot of CO2 and lactic acid that needs to be purged afterwards. I also breathe pure oxygen, both to recover quicker and to prevent any minor decompression problems."

To celebrate the dive, he plans to enjoy a few beers with his safety crew and team - back on dry land of course.