THAM LUANG CAVE, Thailand — Even just reaching 12 boys and their soccer coach trapped in a cave in northern Thailand required a six-hour underground journey that is grueling and treacherous even for the most experienced cave divers: swimming in pitch blackness and vicious currents, squeezing through two-foot-wide passages and climbing over boulders several stories high.
One veteran diver, a former Thai Navy SEAL, lost consciousness and died early Friday after placing spare tanks along the route. Meanwhile, oxygen is starting to run low in the remote cavern where the children have taken refuge.
Three of those in the cavern are reported by the authorities to be weakening, and despite a round-the-clock pumping operation, the threat remains that monsoon rains could push water levels in their precarious refuge even higher.
The initial euphoria in Thailand and around the world that all 13 people had been found alive has given way four days later to deep anxiety over the challenge of getting them out. The option of waiting months until seasonal floodwaters recede now seems less promising, but the practical problems of ferrying 12 children and one adult safely through a nearly three-mile maze of perils remain daunting, all the more so since none of the children are said to be able to swim, much less use diving gear.
“When we found the boys, we thought that the boys would be able to survive in there for a long time,” the Thai Navy SEAL commander, Rear Adm. Arpakorn Yookongkaew, told reporters on Friday. “But now, things have changed. We have limited time. We have to work hard.”
The oxygen level in the boys’ cavern is about 15 percent and decreasing, he said, which is cause for concern: Below 16 percent can cause hypoxia, which in extreme cases can be fatal.
So the rescue effort has grown more urgent. On Thursday evening, rescuers began running a hose toward the cavern in hopes of pumping in more air, in addition to carrying in air tanks for future use, as the diver who perished, Saman Gunan, 38, had been doing.
Divers are also working to run a communications line to the cavern so that officials can better coordinate the rescue attempt and allow the boys some contact with their families. As it stands now, messages must be sent in and out with divers, who risk an arduous 12-hour round-trip journey from cave mouth to the cavern and back.
Interviews with the most experienced of the 140 or so cave divers from Thailand and around the world who are here to help have centered on a stark fact: This was already one of the most difficult cave-diving challenges in the world, and now they must somehow keep the weakened boys reasonably healthy in oxygen-depleted air while trying to teach them to attempt an underwater escape. One cave diver called it the underground equivalent of climbing Mount Everest — but with no guides to make things easier.
Ben Reymenants, a Belgian cave diver who operates a dive shop in Thailand, was part of the group that first found the boys on Monday, after more than a week of searching. He said the muddy current pushing against him on his initial dive felt as powerful as the Colorado River’s.
“You’re literally pulling yourself, hand over hand, in zero visibility,” Mr. Reymenants, 45, recalled in a telephone interview. “You can’t read your depth gauge, you can’t read the time, so you’re basically flying blind in a direction you don’t know.”
Mr. Reymenants said he and other experienced cave divers initially thought finding the group would be impossible under such terrible conditions.
But after it was clear that Thai Navy commanders would continue sending their SEAL members in, Mr. Reymenants said he had volunteered to dive a second time.
“Those kids were at the age that they could have been my son,” he said. “A Navy SEAL can’t just sit there while these kids die in the cave. They have to show some activity — and if you’re a Navy SEAL, yes, you’ll sacrifice yourself.”
More than 110 of the divers are Thai SEAL members, and they have set up a command center in a dry area of the cave known as Chamber Three, where crews are based around the clock. It is about a mile from there to the boys, but it is the hardest mile. Most of it is underwater with few air pockets.
“All is water and dark,” Admiral Arpakorn said. “There are many alleys, up and down. We can say this mission is very brutal.”
One American cave diver, an Air Force rescue specialist who is part of a team sent to help from Okinawa, Japan, said that bringing the boys out now would require shepherding them through underwater passageways as much as a quarter-mile long without air pockets above.
The cave complex, which has never been fully mapped, has many different formations, said the American, who could not be identified by name for security reasons.
It is not a single river running through the cave, he said, and not all of the waterways appear to be directly connected. Pumping water from spots near the cave entrance does not necessarily reduce the level in more distant parts of the network, like where the boys and their coach are.
Underwater, everything is 10 times as difficult as it would be above ground: communicating, solving complex technical problems, providing emergency care, just moving around, he said.
The terrain varies from one area to the next — from sandy bottom to deep mud to boulders the size of a house. In one place, waters converge to create occasional geysers.
Currents can flow quickly, especially when it has been raining outside and the water level in the cave rises.
In some places, he said, one can see waterlines high on the walls of the cave — much higher than today’s levels — showing how high the water has risen in the past.
Some passages are excruciatingly narrow — as small as 2 feet by 2½ feet, Mr. Reymenants said. But the circumstances compelled him to explore the cave in a way that was risky even for a professional who had dived in dangerous spots across the globe, he said.
“Normally, I’d just turn around,” he said, “but then normally I don’t have 12 boys, and their entire lives, as an endpoint.”
Even as the divers and rescue officials navigate the challenges of that environment, concern over the depleting oxygen in the boys’ cavern has become a main concern, Thai officials said.
The governor of Chiang Rai Province, Narongsak Osottanakorn, who is overseeing the search-and-rescue operation, said Thursday night that three people in the cave were getting weaker, although they remained in reasonably good condition.
One of the three is believed to be the coach, Ekkapol Chantawong, who is said to have given his share of the meager food supply to the boys during their 10-day ordeal before they were found.
Reduced oxygen can also cause serious problems. Dinko Novosel, the president of the European Cave Rescue Association, said in a telephone interview that with an oxygen concentration of 15 percent or less in a cave — roughly where it is now — “You can survive, but you cannot walk around or do anything. It’s like being in the high mountains.”
Early Saturday, Mr. Narongsak said officials had decided not to try moving the group out of the cave yet because the boys were not ready for the challenging undertaking.
Admiral Arpakorn said divers would continue the work that Mr. Saman had been doing, bringing in air tanks and placing them at designated points along the route to the group’s cavern.
King Maha Vajiralongkorn Bodindradebayavarangkun, who has closely monitored the rescue operation, directed that Mr. Saman receive a royal-sponsored funeral and that his family be taken care of.
A video clip shared widely on Twitter showed Mr. Saman wearing sunglasses as he stood near the steps of an airplane.
“We will bring the kids home,” he said.
Richard C. Paddock and Muktita Suhartono reported from Tham Luang Cave, and Mike Ives from Hong Kong.