In the final of our series, Paul Charman discovers that leading freediver William Trubridge not only has dangers to contend with but some strict rules if he is to break his own world record next week.

Next week's attempt by Kiwi freediving champion William Trubridge to break his world record for unassisted freediving looks like a quest by a lone man in a vast ocean. But it is no small undertaking.

Trubridge has assembled a team of experts to watch over him as he attempts - on a single breath of air - to swim to a depth of more than 101 metres then return to the surface.

Unassisted by fins or weights, the attempt will be made at Dean's Blue Hole - a huge sinkhole, plunging to a depth of 202 metres on remote Long Island, in the Bahamas.

The small army of people is necessary not just because of safety but also to observe that all the protocols which go towards establishing a world record are adhered to. Freediving may look like one man against the depths, armed only with his lung capacity, but it's a lot more complicated than that.

Viewers of a previous CBS 60 Minutes documentary on Trubridge, may recall his wife Brittany taking charge following an awe-inspiring attempt to dive (with a swim fin - his December 3 attempt is unassisted) down to 124 metres.

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Trubridge reached the depth but muffed his return, because a special protocol is required when the diver breaks the surface. The diver must perform a sequence of activities in order - or the attempt is nullified.

In the final, painful scene of the documentary, Brittany calls to her husband to remove his nose clip, make an okay sign (with thumb and forefinger) and call out: "I'm okay".

However, Trubridge, at his physical limit and suffering from too much carbon dioxide in his system, got confused.

"I was conscious but not 100 per cent lucid and, when that happens, I sometimes forget to remove my facial equipment. I got the sequence wrong and was disqualified by the judges."

He accepted this without question, pointing out that the test is a vital safety measure: "They must check whether the diver has suffered a loss of motor control, as he could be on the verge of blacking out."

The risk of so-called "shallow water blackout" is a potentially deadly hazard to extreme freedivers; Trubridge has experienced it twice. The worst time, in 2006, he blacked out 12m from the surface on his ascent.

"I was brought to the surface quickly by safety divers but it took me a while, maybe 20 seconds, to come round. It was definitely damaging to my sense of confidence."

As well as the disorientation caused by carbon dioxide build-up in the system, extreme freediving carries the risk of nitrogen narcosis. This so-called "rapture of the deep" can befuddle the thinking, cause hallucinations and cloud emotions.

There's even an outside risk of getting decompression sickness (the bends) if a freediver makes repeated deep dives within just a few hours.

Yet despite the potential hazards, and the fact that his support divers can only assist during the final 30m of his ascent, Trubridge, insists freediving isn't nearly as risky as it appears.

"If you look at the statistics, it would probably be one of the safest extreme sports. In terms of deaths during competition, I think we've had one fatality in the world in the last 20 years of competition."

The venue in the Bahamas is also the best place for his world record attempt.

"Probably this is the most accessible blue hole in the Caribbean, with the most dependable conditions," says Trubridge. "It's deep, warm and not much affected by weather. There's no current, or crazy things like jellyfish or great white sharks, so just perfect for what we do."

Just as on December 16, when he set the 101-metre unassisted record at Dean's Blue Hole, Trubridge will be surrounded by his safety crew.

"We'll have five safety divers, a couple of medics and various personnel doing stuff like countdowns and monitoring sonar. Instruments pick up my location and give a read-out of my depth on the surface.

"There'll be several media personnel holding cameras and judges from the International Association for the Development of Apnea (IADA) to authenticate the record.

"It's all a pricey business, so it's great to have Steinlager Pure on board and I hope we get to celebrate with the team following the dive."

Trubridge - who follows a strict training regimen or swimming, breathing exercises, and yoga - says following a record attempt is one of the few times he allows himself to have a beer.

Much production work goes into the filming of his record attempts, which Trubridge admits can be quite theatrical. Having his bother, Sam Trubridge, a Wellington-based theatre director, oversee the filming of previous record-setting dives was highly useful.