Divers successfully rescue four boys but have to leave the remaining eight and their coach behind for now due to low oxygen tank supplies.
They allowed themselves smiles, but there was no cheering among the exhausted rescuers emerging from the Tham Luang cave system.
With the first of the monsoon rains falling on the limestone hills above their heads, Thai Navy Seals and elite cave divers from around the world had pulled off what many thought was impossible - guiding four boys though a terrifying underwater journey in one of the most daring rescue operations of modern times.
The four weakest of the group were selected to be freed first. But the remaining eight could have to wait up to four days to be freed.
With heavy rain setting in at dusk, it was plain to authorities and locals that they now faced a race against time to rescue the remaining eight boys and their 25-year-old coach from their air-pocket prison 2.5 miles inside the mountain, The Daily Telegraph UK reports.
"If we wait and the rain comes in the next few days we will be tired again from pumping and our readiness would drop. If that's the case, then we have to reassess the situation," said Narongsak Osottanakorn, the former Chaing Rai governor who is overseeing the rescue operation.
He added that the operation could only resume once air tanks had been replenished and other systems reinstalled.
Sunday's operation will be remembered as an extraordinary achievement in a drama that has left Thailand and the world holding its breath.
The Wild Boars football players who went missing.
The Wild Boars football team and their coach went missing after heavy rains blocked the exit of the cave complex, which they were exploring after football practice, on June 23.
They were found sheltering on a ledge four kilometres inside the cave on July 2 by Rick Stanton, a former West Midlands fire fighter, and John Volanthen, an IT consultant from Brighton, who are considered among the best cave diver rescuers in the world.
Authorities considered several rescue strategies, including drilling an escape shaft from above or pumping out enough water to enable them to walk out. Diving out was always considered the most dangerous option.
A Thai Navy Seal involved in the rescue died after running out of oxygen in a submerged section of the tunnel last week, and even elite cave divers have described the conditions inside the cave as frightening.
But after weather forecasters warned that fresh monsoon rains were imminent, rescuers realised they had no choice but to act quickly or risk seeing the boys drown.
"Today is the D-day. The boys are ready to face any challenges," Mr Narongsak announced earlier on Sunday morning. "A new storm is coming. If we wait and rain water comes in, our readiness will be lower than now.
Onlookers watch and cheer as ambulances deliver boys rescued from a cave in northern Thailand to hospital in Chiang Rai after they were transported by helicopters. Photo / Getty Images
Thirteen international divers set off to reach the 12 boys and their coach at 9am on Sunday morning. They were supported by 90-strong team of Thai Navy Seals and elite divers from countries including Britain, Australia, the US and China.
They then escorted four of the boys, who have been given rudimentary diving training, on a route that required them to swim for a full kilometre underwater.
The boys were equipped with "positive pressure" full-face diving masks designed to prevent water leaking in and clung to a guide rope to make sure they did not panic or lose their way in water so dark it has been likened to "cold coffee".
The team had to squeeze through gaps just 38 centimetres wide, where the rescue divers were forced to remove their air tanks and push the equipment ahead of them.
Thai media named the first boy out as 13-year-old Mongkol Boonpia. However, his mother, who has been camping by the cave entrance, said she had not been informed by authorities whether her son was among the four rescued on Sunday. She planned to stay at the cave site overnight.
Rescue workers along the main road leading to Tham Luang Nang Non cave as the first 2 ambulances carrying 2 boys pass by on July 8, 2018 in Chiang Rai, Thailand. Photo / Getty Images
At least two of the rescued boys were helicoptered to the Chiang Rai Prachanukroh Hospital, 37 miles away. A third was being treated at a Navy Seal field hospital at the cave entrance.
While little has been revealed about the medical condition of the survivors, their ordeal is likely to take both a psychological and physical toll. Medical staff involved in the mission said they prioritize checking on the boys' breathing and signs of hypothermia. But they could also be looking for deadly diseases known to afflict miners and underground explorers.
"Cave disease", an airborne lung infection caused by bat and bird droppings, can be fatal if it is untreated. Each boy will have a devoted medical unit consisting of at least one doctor, two nurses, a paramedic and an ambulance.
It is understood that 13 helicopters - one for each boy - have been placed on standby to fly the survivors to hospital.
