On a hot day in June 2018, 12 boys and their football coach headed down to the Tham Luang Nang Non cave in northern Thailand for a picnic. The cave was like a playground for the local children; they would go there to explore after practice before cycling home for dinner. But not this time. When early monsoon rains caused the cave to flood, the children and their coach found themselves trapped in a pitch-black chamber four kilometres deep. As the world’s media descended and the drama unfolded over 18 nerve-wracking days, few would have guessed that two gruff blokes from Britain would be the ones to rescue them.

But one man, at least, was not surprised. “We knew the mission had our names all over it,” says 60-year-old former firefighter Rick Stanton. He and 50-year-old IT consultant John Volanthen, who both cave dive for fun, were the first divers to reach the children. “We’re used to going into caves all over the world and we’d been involved in other rescues,” Stanton continues. “We knew we were among the best-placed people in the world to go there and make a difference. But why would you suppose that two British middle-aged men would do the rescue? That was the strange thing. But we were really relieved when we got the call.”

The unassuming pair are at the centre of The Rescue. The captivating new documentary uses extensive interviews, Royal Thai navy footage and underwater re-enactments to tell the extraordinary story of how – after two and a half agonising weeks – all of the boys, some as young as 11, and their 25-year-old coach were saved from the cave. One Thai navy seal, Saman Gunan, suffocated to death while trying to rescue the children, and his story is movingly told in the documentary. Another, Beirut Pakbara, died the year after the mission due to a blood infection he contracted while in the cave.

The film is made by Elizabeth Chai Vasarhelyi and Jimmy Chin, the directors of the Oscar-winning climbing documentary Free Solo. “As parents of Asian kids, Jimmy and I really followed this story,” says Vasarhelyi, who is married to Chin and has two children with him. “We were struck by how so many people from different walks of life and countries and languages came together to really do something that seemed absolutely impossible. Like many people, I didn’t think the children would survive. I mean, it was 10 days before they were found. The whole world was watching, and I think that’s because it’s a story that reminds you of your common humanity. It was riveting.”

Vasarhelyi’s documentary is utterly riveting, too. It maintains suspense despite the fact that we all know how the story ends. This is partly down to the fascinating details it unearths – for example, the scrap of paper upon which Chiang Rai-based caver Vern Unsworth scribbled down the names of the best divers in the world. While the Thai royal navy were on the scene from the start, and their efforts helped lead to the safe rescue of the boys, people with lifetime caving experience were needed. It was on that scrap of paper, hastily handed to a Thai minister, that Stanton and Volanthen’s names appeared.

“Suddenly, this pastime they’d done at the weekends for 40 years became a calling,” says Vasarhelyi. “It prepared them for this moment. They didn’t hesitate. They put everything on the line. And they really thought that saving just one child would be a success. Their moral courage really moved me.”

During the men’s first dive through the cave’s murky waters, they stumbled upon four adult pump workers who’d become trapped. No one had noticed they were missing. As the divers carried them out underwater, the pump workers panicked so much that they nearly drowned. Their journey was just 30 seconds and they were grown men. The realisation dawned that, if the divers were to succeed in finding the boys deeper inside the cave system, they would have to sedate them to get them out.

The footage of Stanton and Volanthen finding the children on day 10 is staggering. After he emerges from the water to find the boys huddled on a rock, Volanthen can be heard in the video saying: “Believe, believe.” He wasn’t addressing the children or Stanton – he was talking to himself, unable to comprehend what he was seeing. Stanton says that moment was surreal. “I was looking at him incredulously, thinking, ‘They’re all here in front of us, why are you saying that?’” he recalls. “I was stunned by his response. But we had assumed all the boys would be drowned.”

When the divers came back to the mouth of the cave, the next question was how to safely sedate the boys. An Australian anaesthetist and cave diver, Dr Richard Harris, concocted a sedative that could be used on the children – a cocktail of Xanax, ketamine and atropine – so that they could be carried underwater in scuba gear, without waking up, for the two-and-a-half hour journey. Harris’s interviews are among the most moving in the documentary, his eyes filling with tears as he recalls pushing the unconscious children’s heads under the water. “It felt very wrong,” he says. “It felt like euthanasia.”

The divers had a complicated mission ahead of them. They would have to top up the children’s sedatives themselves mid-journey, as one dose alone wouldn’t last the whole way. They brought down body bags with them “in case things did go south”.

The rescue itself took place across three days, with Stanton and Volanthen aided by an international cave diving team. They had to be quick – air supply in the cave was dwindling and more monsoon rains were in the forecast. “We’d set up this plan and practised it in a swimming pool, of how we were going to carry the boys, as if they were inert packages,” says Stanton. “But once I got there and saw each child being sedated and pushed under the water unconscious, that brought it home. They weren’t really packages, they were people. Even though we’d planned it to the nth degree, it didn’t really hit us that that’s what we were doing until we were actually doing it. Even to this day, knowing the outcome and having made the plan ourselves, it just seems so preposterous that it worked. It was a preposterous plan.”

One of the largest obstacles in making the film, says Vasarhelyi, was securing footage that the Thai navy seals had taken inside the cave. The pandemic had made it much harder to build trust and rapport with the Seals. “We pursued the footage for two years over Zoom,” she says. “There are just some things you can’t achieve remotely. I finally went to Thailand in April 2021 and they agreed to share it. The material is astonishing. All those critical moments were actually documented in some way: when Rick and John first found the children, the motivational cheer, the children eating for the first time, the blankets, when Dr Harris anaesthetised a child, the very end of the rescue when you see 200 people handing each other stretchers with the children on them.”

After the last child was out, the rescue team cracked open a bottle of Jack Daniel’s in the cave. “We couldn’t let our guard down until the last boy,” recalls Stanton. “It was exceptionally risky. Until we got the last one out, nothing was certain. It was fantastic knowing we’d actually done it.”

Another challenge in making The Rescue was that after the boys were saved, a bidding war took place over the rights to the story. Netflix ended up with the rights for the boys’ accounts and National Geographic, which made The Rescue, got the rights for the divers’. For that reason, the documentary’s focus is entirely on them and their backgrounds.

Stanton’s story, which he has written about in his book Aquanaut, is a tough one. The boys in the Thai cave survived, but he has worked on less successful missions. He has removed dead bodies from caves before, “about nine times” he thinks. One of them was his best friend’s. “Pretty much all of them I knew,” he says. “It’s a very small community. I’m used to stumbling across bodies. Don’t forget, I was a fireman, too.”

He has never sought therapy to deal with the trauma he’s encountered. “You just get accustomed to it and talk it through with your peers in the fire service or the rescue team,” he says. “It’s sad, of course, and tragic, but you’re always analysing what happened and working out a way to avoid it happening to you.”

The Thai cave rescue has had a huge impact on his life. He was a local celebrity in Coventry when he first returned from Thailand, and as well as releasing his book, he’s been working with the documentary makers and the Hollywood film producers. “Not one day has passed in three and a half years where I haven’t had something to do with Thai rescue in some way shape or form,” he says. “It certainly has been a huge feature in my life.”

Vasarhelyi says it’s “bittersweet that the film’s essence – the idea of doing the right thing, being generous – has only become more poignant through the pandemic”.

“I just hope the film, and the lengths these divers went to in order to save 13 strangers, reminds audiences of that common humanity,” she adds. “It’s about being connected in these politically and physically separate times.”

National Geographic film ‘The Rescue’ is out now in UK cinemas