At least four of a group of boys trapped inside a cave in northern Thailand for two weeks have been been brought out, say the Thai Navy Seals.
Rescuers decided to go ahead with the hazardous operation on Sunday because of fears of rising waters.
Divers are guiding the 12 boys and their coach through darkness and submerged passageways towards the mouth of the Tham Luang cave system.
They are being rescued in groups. It is unclear how long the mission will take.
However, it is moving much faster than officials had earlier predicted.
"The fourth Wild Boar is out of the cave," said the Thai Navy Seals in a Facebook post, referring to the name of the boys' football team.
They said they were out by 19:47 local time (12:47 GMT).
The BBC's Dan Johnson, who is at the scene, said doctors went to assess the boys on Saturday and decided on a priority list, sending the weakest out first.
What is happening at the cave?
A huge volunteer and media operation has built up around the mouth of the cave over the past week.
But early on Sunday, journalists were told they had to move down the road, sparking speculation that a rescue mission was about to begin.
Narongsak Osottanakorn, who has been leading the operation, then confirmed that 18 divers had gone in to get the boys.
"This is D-Day," he said. "The boys are ready to face any challenges."
He added that the boys had all been assessed by a doctor and were "very fit physically and mentally... They are determined and focused".
The group and their families had all given their agreement that they should be moved as soon as possible, he said.
How are they bringing them out?
Getting to and from where the boys are has been an exhausting 11-hour round trip even for the experienced divers.
The process includes a mixture of walking, wading, climbing and diving - all in complete darkness - along guide ropes already in place.
Wearing full-face masks, which are easier for novice divers than traditional respirators, each boy is being accompanied by two divers, who will also carry their air supply.
The toughest section is about halfway out - at a section called "T-Junction", which is so tight the divers have to take off their air tanks to get through.
Beyond that a cavern - called Chamber 3 - has been turned into a forward base for the divers. There, they can rest before making the last, easier walk out to the entrance. They are expecting to be taken straight to hospital in Chiang Rai town.
He lost consciousness and could not be revived. His colleagues have said they "will not let the sacrifice of our friend go to waste".
In an indication of quite how dangerous the journey will be, a former Thai navy diver died in the caves earlier this week. Saman Gunan was returning from a mission to provide the group with air tanks.
Officials had originally thought the group might have to stay where they were until the rainy season ended - that could have meant months underground.
They had also been exploring whether they could drill down into the cave, as well as scouring the mountainside for another way in.
But with the rainy season just beginning, it has become clear that the flooding which originally trapped the boys will only get worse in the coming days.