It was the evening of 2 July 2018 when British divers Rick Stanton and John Volanthen emerged from the murky, dank monsoon rainwater that had flooded Thailand’s Tham Luang Nang Non cave system, immediately realising they had found what they were looking for.

Twelve young football players and their coach had been trapped underground about two kilometres into the cave for 10 days by this point, and both Stanton and Volanthen had mentally prepared themselves to find bodies in the water.

But one by one, the divers’ torchlight slowly made out 13 desperate faces in a pitch-black chamber. “Thank you,” the exhausted boys can be heard telling the pair, in footage that was later beamed round the world. “Thank you, thank you.”

This hugely emotive scene is one of the many dramatic moments featured in The Rescue, a new National Geographic documentary that chronicles the against-all-odds story that transfixed the world in 2018. It turned out to be part of hours of footage, the majority of which had been kept locked away.

“When I started filming, I was trying to literally capture the faces of the boys that were still alive so we could work out which ones hadn’t survived,” says Volanthen, speaking to Sky News alongside the documentary’s co-director Elizabeth Chai Vasarhelyi, about the moment he and Stanton found the boys.

“But as we continued counting and filming, it became clear we got to 13 and they had all survived. I simply couldn’t believe it. The situation seemed incredible, that they’d found somewhere out of the water, they’d looked after each other and albeit they were weak, they were in such good spirits.”

Millions of people followed the headlines as the story unfolded, but with the accounts from the divers’, the Royal Thai Navy SEALs and US Special Forces personnel who also worked on the operation, The Rescue really gives a sense of the enormity of what was accomplished in getting the boys out, that there was more to this than even the news reports of a “miracle” conveyed.

It is a story of “great decency and great moral courage”, says Vasarhelyi, a story of the risks some people are willing to take to save a life.

Finding the boys alive was the easy part – what came next was tougher than they could ever have imagined

Stanton, Volanthen and a group of other cave divers, including several from the UK, were called on as they are the best in the world, a group of people with a set of unique skills that even the Navy SEALs – used to diving, yes, but in open water – did not possess.

The documentary shows the team being assembled, presented as a motley group of misfits, almost, who didn’t really look the part, but slowly were able to convince authorities they were the best people for the job; almost like Bruce Willis and his oil diggers being sent to space to stop the asteroid in Armageddon.

But cave diving wasn’t a job, it was a hobby. Although Volanthen had taken part in rescues before, even recovered bodies before, he had never dealt with anything like this.

Finding the boys turned out to be the most straightforward part of the mission. Now they had to work out how to get them out – and there was no way they would be able to make the two to three hour journey underwater without panicking.

After giving the boys a motivational speech to keep spirits up, Volanthen and Stanton had to leave them behind to make their way out and deliver the good news that they were all alive – and work out what to do next. “I did make them a promise that I would come back,” Volanthen says. “Rick felt that was quite rash at the time, but I genuinely meant it.”

Several options were discussed by authorities above ground. Could a tunnel be dug into the cave? Could they wait the three months or so until monsoon season was over?

But dwindling oxygen meant the rescue had to happen quickly. And the only way to do this, Stanton concluded, was to sedate the boys and their coach. The magnitude of this decision – eventually reluctantly agreed by Australian Richard Harris, another of the cave divers who was also a doctor – is difficult to comprehend. Nothing like this had ever been attempted before.

‘None of the options were good’

The documentary features previously unseen footage of the rescue that Vasarhelyi and her co-director, husband Jimmy Chin (they are the team behind Oscar-winner Free Solo) fought hard to include. After negotiations, they managed to secure it, thinking they would have about an hour’s worth to view. In fact, there is 87 hours of footage that exists.

“Until you actually see someone bind a child’s hands behind their back and bind their feet together and push their head underwater, you know, that’s when the emotional gravity and the stakes of what [the divers] were attempting really came home for me,” says Vasarhelyi. The footage also shows the end stages of each child emerging from the water, with hundreds of people supporting the effort to lift them out of the mouth of the cave.

“[When] you see 200 people passing a child and you see the child’s face underneath the mask, it just makes it very clear the scope and the size and the scale of this rescue and how and why you needed so many people to support these 10 divers, and how so many different people came together to do this impossible task.”

The boys and their coach were given a combination of Xanax, ketamine and atropine. Dr Harris had no idea if it was going to work. But it was their only option.

“None of these options were good, but that was what we were faced with,” says Volanthen. “I was incredibly uncomfortable and I would have liked, if I’m honest, to abdicate the responsibility. But I genuinely felt that Rick, myself and the rest of the dive team, we provided probably the best option the children had for life.

“At the time, I wasn’t expecting to be 100% successful; I think it was the American military that said if we save one out of 13, that will be considered a success. Now there’s no way, of course, we would have continued with the same process, not succeeding. But that gives some idea of the unusualness and the severity of the situation.”

Telling this part of the story was bittersweet, says Vasarhelyi. “Doctor Harris, he would never practise again if he had killed one of those kids… and he was fully aware of the psychological risk, everyone was. But it was the right thing to do to try because they were really the only people in the world who could.”

‘Being fearful isn’t a bad thing’

The divers were each assigned a boy to pull through the submerged tunnels of the cave system. The film depicts how hazardous and uncertain the rescue was – one diver even stabbed himself with a ketamine syringe, which fortunately wasn’t connected.

They are all used to being in confined spaces underwater and come across as calm, matter-of-fact. One diver at the end talks about how the documentary shows that being unemotional can be a good thing. But Volanthen says that didn’t stop them being scared when they had other lives to consider.

“I was fearful every second that I held the boy underwater. There’s absolutely no question I was fearful from the second we submerged… till the second when we handed the boys over to the American military and asked the question, ‘they’re still breathing, aren’t they?’ ‘Yes.’ Then relax. Perhaps the risk to our lives was something that we’re used to, perhaps not extraordinary, but the risks to the boy’s life… each of those boys, they were in mortal danger every second they were underwater.

“So I was very much aware of my responsibility. And being fearful of something isn’t a bad thing because that fear can be awareness. If you weren’t fearful for that boy’s safety, then something was very wrong. And we were all equally diligent and all equally able to bring those boys out in the end.”

Vasarhelyi went to Thailand to see the cave herself. Although their accounts do not feature in the film, she also met some of the boys during her research.

“It was a very important experience to me, one being able to walk the cave and understand again the sheer scale of the operation, and also understand how enchanting the cave is,” she says. “I know exactly why those boys went in there. It’s fun, it’s a great adventure. And it was like their backyard.

“I was left with this one kind of anecdotal impression from the children where they talked about how they would role play [while they were trapped in the cave] and pretend to be each other’s parents, when they really missed their parents, and that just brings tears to my eyes every single time.

“Those children are very strong. I mean, they had tough lives. And I think that definitely contributed to their ability to survive.”

National Geographic documentary film The Rescue is out in cinemas now