With time running out amid the threat of monsoon rains in northern Thailand, rescuers were forced to put a complex plan in motion to save the members of the youth soccer team stranded in a subterranean cave system. Last night their efforts were being described as “one of the greatest rescues in history” as the first of the 12 trapped boys were brought to the surface after spending more than two weeks underground.

The only way to get to the group was by navigating 2.5 miles of dark and tight passageways of the Tham Luang cave system, some of which is filled with muddy water and strong currents, as well as oxygen-depleted air. When the rescue operation started in earnest, it was incredibly swift compared to the anguished wait in the days before for the affected families.

Four of the children had been brought to safety as of last night in the rescue’s first phase, with the boys reported to have made the final part of the journey on foot. They were said to be in good health but were taken to a hospital in Chiang Rai province for examination. Rescuers then had to pause for at least ten hours to rest and refill air tanks.

An ambulance carrying one of the boys rescued from Tham Luang Nang Non cave heading towards the hospital. Picture: Linh Pham/Getty Images

Experienced cave rescue experts had considered an underwater escape to be a last resort, especially with people untrained in diving as the boys are. The path out was considered especially complicated because of twists and turns in narrow flooded passages. Those fears were compounded when former Thai Navy Seal Saman Gunan passed out and died making the dive on Friday.

But Chiang Rai acting governor Narongsak Osatanakorn, who is heading the operation, said that mild weather and falling water levels over the past few days had created a window of opportunity for the evacuation.

The operation to rescue the boys, aged 11 to 16, and their coach, 25, by having them dive and swim out of the flooded cave, started yesterday morning. Thirteen foreign and five Thai divers are taking part in the rescue, part of a 90-strong team of divers involved in the operation overall. Two divers are accompanying each boy as they are gradually brought out of the cave system.

Mr Osatanakorn said “the operation went much better than expected”.

A photo on the Thailand Navy SEAL Facebook page shows rescuers hands locked with a caption reading 'We Thai and the international teams join forces to bring the young Wild Boars home'. Picture: Thailand Navy SEAL Facebook page via AP

Peter Faulding, founder of rescue response provider Specialist Group International, said the right team of people were handling the operation, but warned of the dangers faced by the divers and the children.

He said air supply for everyone involved would be one of the biggest issues to deal with, given the length of the journey in and out of the cave.

Mr Faulding predicted it would take four hours to get in and four hours to get out as he said it appeared experts could pull off “one of the greatest rescues in history”.

“It’s a highly dangerous operation – I can’t stress that enough,” he said. “If it goes wrong, there’s no way out. If they run out of air, you have got a big problem. It’s very, very dangerous. There’s no second option really.” The entire operation to rescue all 13 could last two to four days.

“It will be one of the greatest rescues in history,” he added.

Authorities have said incoming monsoon rains that could send water levels in the cave rising, coupled with falling oxygen levels in the enclosed space, added to the urgency of getting those trapped out. Earlier efforts to pump out water from the cave have been set back every time there has been a heavy downpour.

Authorities had ordered the throngs of media gathered at the cave from around the world to leave before announcing the rescue was under way.

The boys and their coach became stranded when they went to explore the cave after a practice game on 23 June. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost ten days.

The search and rescue operation has involved international experts and rescuers. Two elite British divers, Rick Stanton and John Volanthen, were the first rescuers to reach the group on Monday night and are believed to be part of the “crack” team.

Mr Osatanakorn said experts had told him water from fresh rain could shrink the unflooded space where the boys are sheltering to just ten square metres.

“I confirm that we are at war with water and time from the first day up to today,” he said. “Finding the boys doesn’t mean we’ve finished our mission. It is only a small battle we’ve won, but the war has not ended. The war ends when we win all three battles – the battles to search, rescue and send them home.”

The boys sounded calm and reassuring in handwritten notes to their families that were made public on Saturday. The notes were sent out with divers who made an 11-hour back-and-forth journey to act as postmen.

One of the boys, identified as Tun, wrote: “Mom and Dad, please don’t worry, I am fine. I’ve told Yod to get ready to take me out for fried chicken. With love.”

Mick wrote: “Don’t be worried, I miss everyone. Grandpa, Uncle, Mom, Dad and siblings, I love you all. I’m happy being here inside, the Navy Seals have taken good care. Love you all.”

The most touching note came from one of the boys whose name was not clear. It read: “I’m doing fine, but the air is a little cold, but don’t worry. Although, don’t forget to set up my birthday party.”

Another asked their teacher not to give them a lot of homework.

In a letter of his own, the coach, Ekapol Chanthawong, apologised to the boys’ parents for the ordeal. “To the parents of all the kids, right now the kids are all fine, the crew are taking good care,” he wrote.

“I promise I will care for the kids as best as possible. I want to say thanks for all the support and I want to apologise to the parents.”

Three Navy Seals have been with the boys and their coach, one a doctor. The 13 were having health evaluations and rehabilitation and were being taught diving skills.

Food, electrolyte drinks, drinking water, medicine and oxygen canisters had been delivered to them.

Thai Cave rescue described as '˜one of the greatest in history'