Tham Luang caves, Thailand (CNN) The operation to rescue the remaining boys and their coach from a flooded cave in northern Thailand was expected to resume Monday morning, but heavy rain threatened to further complicate the mission.

Four of the 12 boys were extracted from the Tham Luang cave network in the Chiang Rai province Sunday evening, before the operation was suspended overnight to allow oxygen tanks to be refilled before rescue teams begin the next phase.

The four boys, who were taken to a Chiang Rai hospital for further medical examination, are said to be in good health, with their condition described by officials as "not that bad."

But rescue teams don't have an abundance of time. Rain began to fall on Sunday, and more rain is forecast throughout the coming days, which could undo the ongoing efforts to drain the flooded caverns where the other boys remain trapped.

"We have two obstacles: water and time," said Chiang Rai Gov. Narongsak Osotthanakorn earlier Sunday, as rain began to fall across the site near the cave entrance.

"This is what we have been racing against since day one," he said. "We have to do all we can, even though it is hard to fight the force of nature."

'A very smooth operation'

Sunday's mission went quicker than it had in drills over the last several days, according to Osotthanakorn.

Previously, the entire round trip through the cave network was thought to take about 11 hours. But the first of the four boys emerged from the cave entrance about nine hours after a team of 18 international cave diving experts went in to retrieve them.

An ambulance leaves the Tham Luang cave area after divers evacuated some of the 12 boys trapped with their coach for 15 days.

The boys wore "full face masks and the rescue divers carried them out through the passage in the cave complex," Osotthanakorn said in a news conference after the rescue.

"It was a very smooth operation today," he added.

While the governor would not confirm the identities of the four boys, he said the first one emerged at 5:40 p.m., followed by the second boy 10 minutes later. Two other boys emerged from the cave at 7:40 p.m. and 7:50 p.m.

Twelve boys -- aged between 11 and 16 -- and their coach, were discovered by two British divers on July 2, nine days after they abandoned their bicycles and disappeared into the Tham Luang Nang Non cave complex.

Pressure mounting as rain dominates forecasts

Rescuers have a dwindling window of opportunity, with forecasters predicting the return of heavy monsoon rains in the coming days, effectively sealing off the cave until October.

It was the rain that stranded the boys and their coach inside the cave to begin with, after they ventured into the cave network last month.

"The heaviest rain has yet to come," said CNN meteorologist Allison Chinchar. Rain is forecast for at least the next three days.

Rescue teams have been helped by the fact that the rain stopped for several days, allowing water to be pumped out of the cave and making it possible for the four boys and specialist rescue teams to make the final leg of the journey on foot.

Osotthanakorn told reporters there would be a meeting Sunday evening to plan next steps, and that authorities want to ensure conditions are stable before beginning the next phase of the rescue.

A dangerous journey



Those still inside the cave are perched on a small muddy ledge 4 kilometers (2.5 miles) inside the cave complex, surrounded by floodwater and with a Those still inside the cave are perched on a small muddy ledge 4 kilometers (2.5 miles) inside the cave complex, surrounded by floodwater and with a limited supply of oxygen.

For the boys -- some of whom can't swim -- the most dangerous part of the journey out of the labyrinth cave system remains the first kilometer, in which they are required to pass through a flooded channel no wider than a person.

During this process, rescuers need to hold the boys' oxygen tanks in front of them and swim pencil-like through submerged holes. Having completed this section, the boys are then handed over to separate, specialist rescue teams, who help assist them through the remainder of the cave, much of which they can wade through.

Initially, officials said the strongest boys would be taken out of the cave first.

But Osottanakorn said on Sunday it would be up to boys and a doctor inside the cave with them to determine the order in which they would exit. That decision would also depend on a medical examination by the doctor.

"I don't know who will come out first," the governor added.

Divers have previously described conditions in the cave network as some of the most extreme they have ever faced.

The decision to move the boys using divers has not been taken lightly. On Friday, a former Thai Navy SEAL died while returning from an operation to deliver oxygen tanks to the cave.

Finnish volunteer diver Mikko Paasi, a long-term resident of Thailand, said the death of the Thai Navy SEAL had changed the mood on the ground and made real for rescuers just how dangerous the mission had become.

"Definitely, you can feel it that it has an effect, but we're moving on. Everyone is a professional so we're trying to put it away and avoid it happening again," he said, adding: "Everybody is focusing on getting these boys out -- keeping them alive or getting them out."

Photos: Photos: Thai soccer team rescued from cave The rescued team attends a news conference in Chiang Rai, Thailand, after being discharged from the hospital on Wednesday, July 18. Hide Caption 1 of 54 Photos: Photos: Thai soccer team rescued from cave Members of the soccer team greet well-wishers as they arrive for the news conference on July 18. Hide Caption 2 of 54 Photos: Photos: Thai soccer team rescued from cave The team poses with a sketched portrait of Saman Kunan, a former Thai Navy SEAL who died on July 6 while returning from an operation to deliver oxygen tanks to the cave. He ran out of air while underwater, an official said. Hide Caption 3 of 54 Photos: Photos: Thai soccer team rescued from cave Thai Navy SEALs pose for a photo after the rescue effort was finished on July 10. One of them was a doctor who stayed with the team for a week after the group was found alive on July 2. Hide Caption 4 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers carry one of the boys out of the cave. Hide Caption 5 of 54 Photos: Photos: Thai soccer team rescued from cave Divers wave through floodwaters in the cave. Each rescue took multiple hours. Hide Caption 6 of 54 Photos: Photos: Thai soccer team rescued from cave Chiang Rai Governor Narongsak Osotthanakorn speaks during a news conference held after the rescue was finished on July 10. Hide Caption 7 of 54 Photos: Photos: Thai soccer team rescued from cave A helicopter transports rescued boys to the hospital on July 10. Hide Caption 8 of 54 Photos: Photos: Thai soccer team rescued from cave People watch and cheer as a helicopter flies toward an airstrip to transport one of the rescued boys to a hospital. Four boys were rescued on July 8, another four were rescued on July 9, and the rest were rescued on July 10. Hide Caption 9 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers leave after all members of the team had been saved. Hide Caption 10 of 54 Photos: Photos: Thai soccer team rescued from cave This photo tweeted by tech entrepreneur Elon Musk shows rescue efforts during the three-day mission. Musk tweeted early July 10 that he had visited the cave and left a mini-submarine there for future use. Hide Caption 11 of 54 Photos: Photos: Thai soccer team rescued from cave Members of the Royal Thai Navy are pictured with the team inside the cave. Hide Caption 12 of 54 Photos: Photos: Thai soccer team rescued from cave The boys received medical attention after being found alive on July 2. Hide Caption 13 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance believed to be carrying one of the rescued boys heads to the hospital on July 10. Hide Caption 14 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers walk toward the entrance of the cave as rescue operations resumed on July 10. Hide Caption 15 of 54 Photos: Photos: Thai soccer team rescued from cave Hospital staff and a police officer are seen during a news conference that was held on July 10. The eight boys that had been rescued on July 8 and 9 were being treated in an isolation ward in a Chiang Rai hospital. Hide Caption 16 of 54 Photos: Photos: Thai soccer team rescued from cave Thai police and military officers use umbrellas to shield a rescued boy before he was transported to a hospital on July 9. Hide Caption 17 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance exits the cave area on July 9. Hide Caption 18 of 54 Photos: Photos: Thai soccer team rescued from cave Emergency responders surround a helicopter on July 8. Hide Caption 19 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue teams arrange a water-pumping system at the cave's entrance on July 7. Hide Caption 20 of 54 Photos: Photos: Thai soccer team rescued from cave Ambulances wait outside a hospital where the rescued boys were treated. Hide Caption 21 of 54 Photos: Photos: Thai soccer team rescued from cave Thai military personnel walk into a cave during rescue operations. Hide Caption 22 of 54 Photos: Photos: Thai soccer team rescued from cave Military personnel work inside a cave. Hide Caption 23 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance leaves the scene of the rescue effort on July 8. Hide Caption 24 of 54 Photos: Photos: Thai soccer team rescued from cave This undated photo, released via the Thailand Navy SEAL Facebook page, shows rescuers with their hands locked. The caption said, "We Thai and the international teams join forces to bring the young Wild Boars home." The Wild Boars is the name of the soccer team the boys play on. Hide Caption 25 of 54 Photos: Photos: Thai soccer team rescued from cave Family members pray in front of a Buddhist statue near the cave on July 8. Hide Caption 26 of 54 Photos: Photos: Thai soccer team rescued from cave Emergency workers carry oxygen tanks as rescue operations begin. Hide Caption 27 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers work to connect pipes that help water from entering a cave. Hide Caption 28 of 54 Photos: Photos: Thai soccer team rescued from cave Thai forest rangers examine a map as they review their rescue options on July 8. Hide Caption 29 of 54 Photos: Photos: Thai soccer team rescued from cave This image, taken from video on July 2, shows the team trapped inside the cave. That was the day divers found them alive. Hide Caption 30 of 54 Photos: Photos: Thai soccer team rescued from cave Divers were able to bring food, blankets and other supplies to the boys while experts determined the best way to get them out safely. The team was found around two kilometers (1.24 miles) into the cave and somewhere between 800 meters to one kilometer below the surface, according to a British Cave Rescue Council briefing note. Hide Caption 31 of 54 Photos: Photos: Thai soccer team rescued from cave The team was found alive by British divers. The search brought rescue teams from all over the world. Hide Caption 32 of 54 Photos: Photos: Thai soccer team rescued from cave A Thai military medic provides first aid to one of the boys. Hide Caption 33 of 54 Photos: Photos: Thai soccer team rescued from cave The body of Saman Kunan, a former Thai Navy SEAL, is carried during a repatriation and religious rites ceremony on July 6. Kunan died Friday as he returned from an operation to deliver oxygen tanks to the cave. He ran out of air while underwater, an official said. Hide Caption 34 of 54 Photos: Photos: Thai soccer team rescued from cave Thai police take a break for breakfast as rescue operations continue on July 4. Hide Caption 35 of 54 Photos: Photos: Thai soccer team rescued from cave Water is pumped out of the cave on July 3. Hide Caption 36 of 54 Photos: Photos: Thai soccer team rescued from cave Workers fix the road leading to the cave on July 3. Hide Caption 37 of 54 Photos: Photos: Thai soccer team rescued from cave Divers prepare a light to be used for the underwater search on July 2. Hide Caption 38 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers wait at the entrance to the cave on July 2. Hide Caption 39 of 54 Photos: Photos: Thai soccer team rescued from cave Some of the team's family members celebrate after receiving news that the team was alive. Hide Caption 40 of 54 Photos: Photos: Thai soccer team rescued from cave Technicians lift water pumps to the drilling site on July 1. Hide Caption 41 of 54 Photos: Photos: Thai soccer team rescued from cave Soldiers and rescuers work outside the cave complex on July 1. Hide Caption 42 of 54 Photos: Photos: Thai soccer team rescued from cave Monks attend a Buddhist prayer for the team on July 1. Hide Caption 43 of 54 Photos: Photos: Thai soccer team rescued from cave Scuba tanks are delivered to the search site on July 1. Hide Caption 44 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers carry water-pumping equipment into the cave on July 1. Hide Caption 45 of 54 Photos: Photos: Thai soccer team rescued from cave A Thai Air Force worker drops near a possible cave opening on June 30. Hide Caption 46 of 54 Photos: Photos: Thai soccer team rescued from cave British cave diver Robert Charles Harper explores an opening on June 29. Hide Caption 47 of 54 Photos: Photos: Thai soccer team rescued from cave Relatives of the trapped team members pray on June 27. Hide Caption 48 of 54 Photos: Photos: Thai soccer team rescued from cave Family members waited more than a week for news of their loved ones. Hide Caption 49 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers bring hoses and additional water pumps as the search for the team continued on June 27. Hide Caption 50 of 54 Photos: Photos: Thai soccer team rescued from cave Volunteer paramedics wait near the cave entrance on June 27. Hide Caption 51 of 54 Photos: Photos: Thai soccer team rescued from cave A rescuer looks for a cave entrance on June 27. Hide Caption 52 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers search the cave on June 26. Hide Caption 53 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers relay cable at the search site on June 26. Hide Caption 54 of 54

Families remain vigilant, hopeful

In the hours preceding the rescue, a letter the boys had sent to their families was published on the Thai SEALs' Facebook page. The letter shows the boys in good spirits despite their ordeal.

In neat blue handwriting, 11-year-old Chanin Viboonrungruang, the youngest of the group, told his parents not to worry, and said he was looking forward to eating fried chicken.

JUST WATCHED CNN talks to father waiting for trapped son Replay More Videos ... MUST WATCH CNN talks to father waiting for trapped son 02:37

His parents, along with other families, have maintained a constant vigil at the site since the boys became trapped.

On reading the letter Saturday evening, Chanin's father, Tanawut Viboonrungruang, said he felt a great sense of relief. "I had been worried about my son, that he would be exhausted, he would be tired," he said.

The aunt and grandmother of one of the boys, Prajak Sutham, said earlier in the day they learned the rescue effort was underway via Facebook, and were anxiously awaiting news that Prajak had made it out safely.

"It's like I'm counting every second," said Salisa Promjak, the aunt. "I want to see his face. I want to see how he gets out, how they get him out ... I am so happy."

Onlookers watch and cheer as ambulances deliver boys rescued from a cave in northern Thailand to hospital in Chiang Rai, Thailand, after they were transpored by helicopter on July 8, 2018.

Asked what she would say when she first sees her grandson, Kiawakham Chantaphoon said, "Grandma loves you the most in the whole world."

They also wanted to express their gratitude to the rescue teams working to free Prajak and his teammates.

"As a cousin and as a representative of all 13 families, I cannot find words to explain our feelings and how glad and grateful we are," said Promjak. "Just the words 'thank you' is not enough to explain all the feelings we have for them."

Rescued boys begin the recovery process

Now that the first of the boys have been taken to a nearby hospital, doctors there will begin evaluating the impact to their long-term health

One of the primary concerns will be the boys' exposure to a lack of oxygen, said Dr. Darria Long Gillespie of the University of Tennessee. "As soon as they get out, that's what they'll be checking: their oxygen levels and their breathing."

Thai officials said Friday that oxygen levels inside the cave plummeted to a dangerous 15% -- shy of the US Department of Labor's Occupational Safety and Health Administration's " optimal range " of oxygen, which is between 19.5% and 23.5%.

As oxygen drops below the lower threshold of the optimal level, the body begins to undergo changes and may face the serious risk of hypoxia, the condition that causes altitude sickness, according to OSHA.

The boys will also be checked for malnutrition, dehydration and an array of other health effects.