The world held its breath.

Sunday, July 8 began like any other morning in Tham Luang cave complex. The Wild Boars woke early, as they always did, in the dark. The air inside the cave was humid, but as they entered their 16th day perched in the Nern Nom Sao cavern, they were feeling the cold, despite the space blankets and extra clothes they'd been given.

After being trapped for so long, the sandy bank on which they were perched felt smaller than ever. It was still only five to six metres wide, and about 20 to 25 metres long from the water's edge to the back of the cave, while the slope, at various points, was as steep as 30 degrees. The conditions in the cave – the darkness, the smell, the constant chill – were wearing them down.

The day before, some of the divers, including the Australian doctor and anaesthetist Richard "Harry" Harris, had visited and given them a detailed account of the rescue plan, which had been accepted by the Boars' families, despite their fears that it could end in disaster and tragedy. The boys were desperate to see their families again, to go home and sleep in their own beds. They were ready to be kids again, to stop being brave, to breathe fresh air, ride around on their bikes, play football and see their friends at school. The day had dawned with the promise of all these things and more. It was time.

After fasting in preparation for being sedated during the rescue, six of the boys had woken hungry. The rescue divers faced dawn that day with a faint feeling of dread. The British team of Rick Stanton, John Volanthen, Chris Jewell and Jason Mallinson, and Australians Richard Harris and Craig Challen, a retired vet, had all been in similar situations. They'd participated in some of the riskiest cave rescues the world had seen. They'd lost friends and undertaken salvage operations; they'd helped bring back the bodies of the dead. Maybe even cried afterwards, when it was all done. They'd also saved lives, brought people back to the surface – people who'd had no hope of being rescued. These guys were the best of the best.

The diving itself may not have been the most technically difficult, the most arduous or the deepest they'd experienced. But nothing like this had ever been attempted before; usually, by the time help arrives, it's already too late. There was no baseline against which to measure themselves. And no one in the rescue team had undertaken a mission like this before, with many millions of people watching, hoping, praying they would succeed.

The mission had been approved by Thailand's king and the government, and reviewed by teams of experts. In theory at least, it had a good chance of succeeding. But there were so many things that could go wrong at any point; they couldn't anticipate everything. The worst-case scenario – that some or even all of the boys would lose their lives – had been war-gamed again and again. If one of the boys woke up, panicked and ripped off his mask in an underwater section of the cave, he could endanger the life of his rescuer, as well as himself.

Everything possible had been done to prepare. For the boys, there were special wetsuits that would ensure they didn't lose too much body heat on the journey out, as well as full-face masks to fit the smallest of them. Oxygen and air cylinders had been placed at strategic points in various chambers throughout the cave.

But for all their careful preparations, most of the rescuers expected multiple casualties. As Challen would later recall, "It wasn't dangerous for us but I can't emphasise enough how dangerous it was for the kids. It was absolutely life and death. We didn't expect to be getting 13 people out of there alive."

Harris was perhaps the most pessimistic. "Personally, I honestly thought there was zero chance of success," he told a conference in Melbourne in late September. "I honestly thought there was no chance of it working. We set up a system for some feedback to come back after the first one or two kids [began their dive out] to me. If they hadn't survived the first sump [a hollow or depression in which water collects in a cave floor], which was the longest one but not the most difficult one, I was going to say, 'That's all I can do', and walk away at that point."

The deteriorating air conditions were one of the key reasons the rescue went ahead when it did. After final meetings were held outside the cave, with Thai officials checking every last detail of the plan, the rescue mission finally began at 10.08am when a team of 13 international divers entered the cave. Heavy rain had fallen overnight and, later that day, it would fall again, but at this hour the skies over Mae Sai were grey and threatening rain, underlining the urgency of the mission.

The 13 divers – Rick Stanton, John Volanthen, Chris Jewell, Jason Mallinson, Richard Harris, Craig Challen, Claus Rasmussen, Mikko Paasi, Erik Brown, Ivan Karadzic, Jim Warny, Josh Bratchley and Connor Roe – were some of the best cave divers in the world. They were closely supported by five Thai Navy SEALs as well as dozens of personnel from the Thai military, the United States, China and Australia positioned in the first three chambers of the cave.

Lights, dozens of air tanks and radios that can communicate with the outside world had been set up in chamber three – as had makeshift medical facilities, so the boys could be checked as they came out.

The area around the entrance swarmed with members of the Thai military, rescuers and medics. Thirteen ambulances were on standby, as they had been for days, while helicopters were prepped and ready to ferry the boys 70 kilometres to Chiang Rai Prachanukroh Hospital, where medical staff waited to receive them. The eyes of the entire world were on this corner of northern Thailand.

About two hours after the rescue mission began, the local governor, Narongsak Osatanakorn, fronted the ever-expanding contingent of international media. The huge mountain range of Doi Nang Non, shrouded in clouds and mist, loomed over the new makeshift media centre and served as a constant reminder that the boys' fate was about to be decided. Narongsak began his press conference with words that would reverberate around the world: "Today we are most ready, today is D-Day. At 10am today 13 foreign divers went in to extract the children along with five Thai Navy SEALs."

A hushed excitement fell over the crowd gathered outside the operations centre as Narongsak continued: "We can say they are all international all-stars involved in this diving operation and we selected five of our best who can work with them. The kids are very determined and they are in high spirits. All 13 kids have been informed about the operation and they are ready to come out. If we don't start now, we might lose the chance."

Although the water levels had receded by as much as 30 centimetres in some sections of the cave, the divers had, at most, a three-day window before the forecast rain would make rescue impossible. Narongsak estimated that the first of the Boars would not emerge from the cave until at least 9pm that evening – 11 hours after the rescue began – and warned that the mission would not be finished in just one day: "It will take time. It's not that we start 10am today and everything is done. We will continue the mission until the last one is out."

Inside the cave, the 13 rescue divers had begun their slow journey through the nine chambers from the entrance to Nern Nom Sao. Most would position themselves at strategic points along the route, while Harris and four British divers – Volanthen, Stanton, Jewell and Mallinson – would travel all the way to the boys, where the Brits would be in charge. Each of the four British divers would take one boy all the way from chamber nine to the entrance. Almost every step of the way, a second diver would assist the man bringing them out.

The Brits, Harris and his dive buddy Challen went in first, as they had the longest dive ahead of them. Travelling in pairs, the six men set out from chamber three, where the diving began in earnest, at about 20-minute intervals. Over the next few hours they slowly made their way to predetermined points along the route. In spite of the lower water levels, the conditions were still very tough.

Challen, Rasmussen and Paasi were stationed in chamber eight, the first stop on the return journey for the four Brits as each came through with one boy. The route from Nern Nom Sao to chamber eight included a 350-metre dive that would be one of the hardest sections to negotiate. The rescue plan called for Challen, a retired vet, to be ready to deliver a "top up" injection to the boys in chamber eight to keep them sedated, if necessary. Harris revealed in late September that the night before the first rescue mission, he held a practice session for the divers involving a plastic drink bottle so they could get an (improvised) feel for what delivering an injection was like. The British rescuers were prepped and ready to deliver top-up injections on the way out, too, and the instruction from Harris was to err on the side of keeping the boys sedated.

In chamber six, Karadzic and Brown would be ready with air and oxygen tanks, and more medicine to inject into the boys if the effects of the sedative were starting to wear off.

Between chambers six and five, there was another 150-metre dive, then a 150-metre canal. And in chamber five, Roe and Warny would be waiting with more air, oxygen and medicine to help the divers and each of the boys through to chamber three. Along the way, there were two more dives of about 150 metres each before, finally, they reached chamber three.

Chamber three was relatively huge – perhaps half the size of a school gymnasium, according to Brown. In all, there were perhaps another 150 rescuers stationed between it and the exit.

In chamber three, each boy's vital signs would be checked by doctors and gauze placed over his eyes to protect them from the light outside the cave. Then he would be placed in a Sked stretcher and loaded onto the elaborate pulley system, or highline, which made it simpler and quicker to transport each boy through to the entrance. But some sections simply couldn't accommodate the highline, so he would still have to pass through a couple of hundred hands as he was brought out of the cave.

In theory, it sounded straightforward. The planning had been methodical, the staging of equipment and men meticulous. The mission was high risk, but it could work. But no one could anticipate, or plan for, how the boys would handle the situation. All the preparation and medication in the world would count for nought if one of the boys woke up mid-dive and panicked. If that happened, Thai authorities would be counting the number of casualties, not lives saved.

Meanwhile, at Nern Nom Sao, the boys were nervous; they knew what, and who, was coming. Although six of the Boars had fasted the night before, it was decided at some point that the divers would attempt to bring out only four boys that Sunday. After talking it through with coach Ek and the Thai Navy SEALs, who had stayed with them since they were found, they'd decided among themselves who would go out first. Volanthen, Stanton, Jewell, Mallinson and Harris had arrived at Nern Nom Sao after swimming and diving for close to two hours. Now it was time to get started.

Richard Harris was ready to prime the needle; it was time to sedate the first Wild Boar. In consultation with army medic Dr Pak Loharnshoon as well as a team of Thai doctors outside the cave, Harris had calculated approximately what dose each boy would need. He would give each of the boys alprazolam – more commonly known as Xanax, an anti-anxiety drug – by mouth, then inject him in each leg with ketamine, a sedative.

Harris had devised a plan for handling the injections. Although he had visited them the day before, and someone had read the boys instructions in Thai, telling them what would happen at the start of the rescue, he didn't want the Boars watching while one of their mates was injected and then submerged. So he asked the Navy SEALs to take the other Boars up to the top of the Nern Nom Sao slope.

The instructions were to the point. First, each of the boys to be taken out that day would swallow a tablet, which would make him feel a bit strange, then he would join Harris at the bottom of the bank, near the water. There, he would be injected in both legs and go to sleep. When he woke up again, he would be in a hospital bed, out of the cave. The boys had listened intently to this plan and nodded along, questioning nothing. As the rescuers went about prepping the four boys one by one in their deliberate, careful fashion, they made the most of the boys' relative ignorance of what lay ahead, using it to their advantage.

While Harris prepared his sedatives, Pak briefed the kids again about the rescue plan. As he spoke to the boys in Thai, the doctor, who had already spent seven nights in the cave with the three SEALs, paid careful attention to whether or not the first four boys were ready for the mission. They were; they were eager to get started. Down by the water line, under unsteady torchlights, Harris was ready. He plunged the needle into 14-year-old Prajak "Note" Sutham's leg; he would be the first boy out. "They seemed to be very confident and having Dr Harris helped a great deal. He was in charge of giving the kids 'medicine' and he was great," Pak says. "He had techniques to talk to the kids, he hugged them and he was so great with them. Dr Harris was like a father or grandfather figure to them."

Harris's calm, reassuring bedside manner was vital in getting the kids to the point where they were ready to dive, and he left nothing to chance. Once the first boy was sedated, and his full-face mask fitted, Harris took the boy down to the water and pushed his head under water. It may have seemed like the wrong thing to do, but it was absolutely critical to test each full-face mask to make sure it fitted properly. After all, better to discover a problem immediately rather than a half-kilometre into the rescue mission.

After about 30 seconds, which passed with agonising slowness, the first Boar started breathing again; the mask worked, and the sedative had been administered in the right dose. Over the next three days, Harris would repeat this breathing test 12 times. The plan was to keep the boys and Ek completely "under" at least as far as chamber three. By the end, Harris had taken to horrifying the Brits with the procedure: "Watch this, he will stop breathing for a second..."

Narongsak would later confirm that the sedation plan was supposed to be kept quiet, but Thai Prime Minister Prayut Chan-o-cha let it slip.

After so long in the cave, a couple of the boys showed signs of the early stages of pneumonia; it was remarkable that more of them weren't ill. But the last boy to leave on that first day gave Harris a scare. After Harris administered the injections in each leg, the boy "behaved like a bad kid with a chest infection under anaesthetic – breath holding, he was over-sedated," he later recalled at a medical conference in Sydney. Harris lay down on the sand with the boy for half an hour, spooning him and listening to his breathing to ensure his airways remained open. It was one of many tense moments over the next three days. Harris later said he was "thinking this is what I predicted would happen, this is going to go really badly. Then he sort of fired up. He ended up needing another dose to put him back [under] in the water about 200 metres down the track."

To minimise the risk to the rescuers on the way out, it had been decided to err on the side of caution and administer two to four top-ups to each boy as he was brought out of the cave. Harris had consulted Thai medics and Australian ones about how to handle the situation, but he was the doctor on site, so he ultimately had to make the call. For the Brits, the sedatives had been non-negotiable. They simply wouldn't have undertaken the mission without Harris, in the cave, administering the doses that put the boys under.

Although the divers knew what to expect, and the water temperature was holding up well, still hovering around 20°C, those first few moments in the water were a shock. Each man had the life of a young boy in his hands; each of the boys was, at best, semiconscious and, even if he had been fully conscious, he would have been unable to fend for himself in this incredibly dangerous place.

It is unclear whether all the boys were completely unconscious. Some of the rescuers, such as Rick Stanton, have insisted they were, and would remember nothing. Ivan Karadzic, however, recalls that at least one had his eyes open, and was speaking Thai – "though my Thai isn't good enough to know what he was saying" – as they came out of the cave. Though he agrees with Stanton that it's unlikely the boys remember much, if anything, about the rescue mission.

Once Mallinson was in the water with the first boy, Note, the pair submerged for the initial 350-metre dive, which took them to chamber eight. As well as the wetsuit and full-face mask, Note was wearing both a buoyancy device and a harness that would keep him attached to Mallinson and also give the diver a handle to grab onto. Note also had an oxygen cylinder strapped to his front, and was positioned beneath Mallinson to ensure he didn't hit his head on the roof of the cave. Mallinson wasn't concerned about his own safety, but he was nervous about the fate of the 15-year-old he held in his hands.

"I was very nervous when we took them from the end point and you got into that first flooded section," Mallinson says. "Until you got a feel for the way their breathing rhythm was going, it was very nervous for the first five or ten minutes, you just wanted to see those air bubbles coming out of that mask all the time."

It was dark in the water, but the lights each diver wore helped him to see. The next step was to locate the path-finding line, which would help him to pull them through. Sometimes, the boy would be positioned to the left or the right of the diver, depending on where the guideline had been laid. At other times, the diver was so close to the boy that he could feel and see the air bubbles that would slowly escape from the face mask his young charge wore.

To get through the narrowest choke points, the diver would push the boy through first with the help of the other divers stationed throughout the cave. It was a painfully slow process, and took much longer than the couple of hours it had taken for them to reach Nern Nom Sao.

It was sometimes impossible to avoid bumping the boys into rocks and other obstacles. The key was to keep the boys' full-face masks on – a task that, as the hours passed, became mentally exhausting. One wrong manoeuvre, one wrong turn could dislodge them. If that happened in a section of deep water and there were still 100 or 150 metres left to dive, the diver would have only a matter of minutes to get the boy's head out of the water and into the open air before he drowned. The oxygen saturating the boys' systems would buy them a little extra time, but not much. The reality was that if a face mask came off, depending on where it happened in the cave, it could be impossible to save the boy, and the diver would have to carry a corpse out of the cave. The extra concentration required to protect these young lives would take a heavy toll on the divers.

In chamber eight, Craig Challen, Claus Rasmussen and Mikko Paasi were enduring an agonising wait for Mallinson and Note, the first boy, to arrive. But as soon as they reached chamber eight, the trio swung into action. First, Challen checked the child's breathing; he was alive and breathing normally. The three men were flooded with relief. Now it was time to start removing Note's diving gear. A muddy, rocky section that was about 200 metres long lay ahead and the team would have to carry the boy on a stretcher then drag it through a section that included a narrow sump that was difficult to negotiate. The pumping that had been going on for more than a week had helped drain this section of the cave of most of its water.

Once they had cleared this section, Challen checked the boy again and the team began to put the kid's diving gear and full-face mask back in place. It was time to go back in the water – another dive, past chamber seven and through the T-junction and on to chamber six.

Sometimes the rescuers had to drag the boy after them. Sometimes, at the narrowest points, they would have to try different techniques to get him through sumps and openings that could be as narrow as 40 centimetres. At other times, a steep vertical climb or dip would present itself.

Although each of the boys had lost an average of a little over two kilograms, it was still difficult to wrangle them through the cave. Every obstacle would eat up precious minutes that raced by. Again and again on that first day it was a case of trial and error as the four British divers grappled with how exactly to get the boys through those first six chambers to chamber three, where a huge rescue team waited for them. At this point the divers had been working their way into and then back out of the cave for at least four hours.

In chamber six, about 100 metres past the T-junction, more help was waiting. Like the trio in chamber eight, Ivan Karadzic and Erik Brown had been sitting in the near darkness for about two and a half hours, although to them it seemed much longer. They were primed and ready for the moment the first boy and his diver would appear and they could check the Boar's breathing. But as Mallinson surfaced and started swimming with Note towards them, the two men were consumed by one fundamental question: was the boy still alive?

They had to face this over and over again, as each boy was brought into chamber six. Had some terrible mishap occurred? Was the diver bringing a dead child towards them?

Miraculously, one by one, the boys came through safely and were all fine. Canadian diver Erik Brown says he will never forget the moment when the first boy came through: "You're not sure what's about to happen, but you're optimistic. You're on edge, in the dark, and you finally see that little light appear. Your heart is going a million miles a minute. When they came through the darkness, that first time, it was in slow motion."

One by one, Mallinson, Volanthen, Jewell and Stanton kept going calmly and methodically – diving through the canals, struggling through the mud and guiding each boy around rocks, through narrow openings, S-bends and sumps, and up rocky slopes. By the time they reached chamber three, where they were greeted by about 150 people, the divers were naturally exhausted. But each man would hug his boy before handing him over to the huge support team.

The atmosphere in the chamber was electric, and there was a murmur of excited voices as the rescuers carried out their tasks as quickly and efficiently as they could. Doctors and nurses checked the health of each boy while the rescue stretchers and the pulley system were made ready. Thai, American, Chinese and Australian personnel and more swung into action, helping the divers to get the boys through the final chambers and out to the waiting ambulances.

Jason Mallinson was the first man out with Prajak "Note" Sutham. He was followed by, in order, John Volanthen, Chris Jewell and Rick Stanton. The other three boys who came out on that first rescue day were 14-year-old Nattawut "Tle" Takamrong, 15-year-old Pipat "Nick" Pho and 17-year-old

Peerapat "Night" Sompiangjai.

The four divers had brought the first four boys to the brink of freedom, defying everyone's expectations as well as the harsh conditions. Just four of them, with nine men in support positions along the way, had swum, dived and dragged the boys for hours, through the hardest sections of the cave. The next day, they would have to do it all again.

Against all odds, all 13 Boars were saved over the next two days, while the brave Thai Navy SEALs who had stayed with the boys after their discovery survived, too – escaping the cave even as the pumps that had held back the tide of water failed. After a week in hospital, some hearty meals of stir-fry chicken with basil, and a short stint as ordained Buddhist monks, the boys returned to their families, school, homework and eventually to the football field. Their lives would never be the same again.

Edited extract from The Great Cave Rescue by James Massola (Allen&Unwin;, $30), out November 14.

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