A dropped guideline, an ill-fitting face mask during final rescue put Thai cave rescuers and the boys in peril "The pressure was really on now," British diver Chris Jewell told "20/20."

It was British Cave Rescue Council diver Chris Jewell’s very first rescue mission, and the entire world was watching.

An elite team of divers, backed by a massive international effort, was entering Day 3 of rescuing a trapped Thai soccer team from deep inside a miles-long cave in Thailand. Things had been going more smoothly than any of the planners had anticipated those first two days of the rescue. But conditions inside the cave were deteriorating. And as rescuers were pulling the last boys and the coach out, it seemed the dangerous mission that had already claimed the life of one former Thai Navy SEAL could claim more.

“The pressure was really on now,” Jewell told “20/20.” “Now actually the expectation was that we would get them all out. … The chance of losing a single child would be catastrophic.”

It was an impossible mission from the start. The 12 boys, between the ages of 11 and 17, had hiked deep into the Tham Luang Nang Non cave with their 25-year-old soccer coach and had been trapped for more than two weeks. The boys were already weak from deprivation and hunger. A trio of Thai Navy SEALS and a medic had stayed with the boys since they had been discovered after ten days in the cave. The British rescue team tasked with diving them through the flooded cave were Chris Jewell, Jason Mallinson, Rick Stanton and John Volanthen. Australian anesthesiologist and rescue diver, Dr. Richard Harris prepped the sedative Ketamine that would be used to put the boys under to ensure that they didn't panic.

“A thick, maybe 9- or even 10-millimeter-thick climbing rope was installed in the cave and we'd use that as a guideline, which we could swim along as well as pull ourselves against the current,” Jewell said.

With each boy, the divers had carried out a choreographed underwater ballet. They would hold onto the guideline that stretched to the cave entrance with one hand and use the other hand to hold onto a strap on the boy’s vest as he floated facedown wearing a face mask attached to an oxygen tank. The divers did this while maneuvering around any obstacles in the darkness, and switching the boys from side to side depending on the hazards they encountered, going from Chamber 9, where the soccer team was found, back to Chamber 3, where other rescuers then brought them to the cave entrance.

On the final day of the rescue, as Jewell was diving the approximately 500 feet from Chamber 4 to Chamber 3 with one of the boys, he lost his grip on the guideline.

“It was moving one of the boys from my left hand to my right hand,” Jewell said. “I managed to let go of the guideline, which cave divers use to navigate in zero visibility.”

Jewell said he made a sweeping motion to try to find that guideline but he couldn’t. It was gone.

Instead, he found an electrical line that the support team had installed before the rescue mission. Jewell grabbed onto that, assuming it would lead him towards the cave’s entrance. But in the darkness, and still hanging on to the boy, Jewell said he became disoriented and headed in the wrong direction.

“I surfaced in a different section of the cave and I really didn't know where I was for several minutes,” he said. “I got the boy out of the water, made him comfortable, took off my cave-diving equipment and then I was able to walk around the chamber and recognize that I was back in Chamber 4.”

Meanwhile back in Chamber 9, Mallinson, and Australian diver Dr. Harris were readying the last boy to dive.

“The last child is there who is a really small child,’ he said.

In fact, Mallinson said he realized the boy was too small for the diving mask that he had brought for him.

“It didn’t fit him,” he said. “We put it on him, really strapped down tight so his nose was flattened against his face and there was a big gap under his chin. We just couldn’t get it to seal.”

This was critical because if the mask’s seal didn’t hold, the rescuers fear, the boy would drown. He had already been sedated so they had to think quickly. Mallinson decided to try a different mask that he thought was the boy’s best shot at having a tight seal.

“It was so nervous for me because it was the different type of mask with this seal that you could dislodge sideway. I had to be so careful with him,” he said.

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With absolutely no communication from Chamber 9 to rescuers down the line, there was no way for Mallinson to get word to the team at the cave entrance to bring him a new mask, but he said it never crossed his mind to leave the boy behind and postpone getting him out.

“We knew we didn’t have any more time and we knew this was the last option,” Mallinson said. “Once you set off with that kid, it was a one-way journey. You weren’t going back to where they started. … It was a case of getting him out. A bit brutal but dead or alive.”

So, he made the decision to begin this last trip, but visibility was deteriorating, which meant he had to move more slowly.

“You could hardly see [your] hand in front of your face,” Mallinson said.

He said he tried to focus on keeping the mask on the boy’s face and preventing the boy’s head from smashing against the cave walls.

“Because the visibility was so bad, I know I couldn't really stop his head banging against the wall because I couldn't see his face or his head,” he said. “So I developed a technique where I'd pull him in really tight with his head just down here. And, I’d extend my head over the top of his so my head hit the wall first and it so protected his head.”

Mallinson estimated that he hit his own head on that third day “dozens” of times, but he knew he had to keep moving. Harris, the anesthesiologist who had administered the sedative, had warned the dive team about hypothermia.

“We were diving, generating heat, so we could keep ourselves warm but we were worried at first that they [the boys] would be hypothermic by the time they got to Chamber 3,” Mallinson said. “That was one of the instructions Dr. Harry gave us. He said, ‘No matter what happens, get them out as fast as possible. Hypothermia is going to be a big issue here.’ So, you know, we went with haste.”

On the way out, Mallinson and Harris came across Jewell, still in Chamber 4.

“Harry [the anesthesiologist] was able to take the boy from me and dive ahead,” Jewell said. “I followed him [Harris] closely behind just making sure that he didn't have any problems in the way.”

When Mallinson, Jewell and Harris finally walked through Chamber 3, having delivered the last boys, they were met with thunderous applause from hundreds of rescue workers.

“We go out to... help people with the skills we've got. That's what we did,” Mallinson said. “We were happy about that. It's nice to be applauded and stuff but it's not what we're after.”

"I don't feel like a hero," Jewell added. "I just feel like someone who was in the right place at the right time with the right skills to make a difference."

As each boy and their coach was brought to a waiting ambulance and taken to the hospital, the rescue divers began to take in what they had just accomplished. A medic and three of Thai Royal Navy SEALs were the final ones to emerge from the cave safely, and it was at that moment that Mallinson allowed himself a hint of a smile.

“I mean we did our best for them. They all came out alive,’ he said.

His advice to the boys – next time they go into a cave, check the weather first.