(CNN) The death of a former Thai Navy diver has cast a shadow over the effort to rescue 12 boys and their soccer coach from a cave in northern Thailand and raised fresh questions about the best way to extract them safely.
More rain is forecast this weekend, putting pressure on rescuers to act soon or have to contend with even higher water levels inside the cramped chambers.
The rescue is further complicated by the lack of oxygen inside the tunnels -- as more rescuers enter to carry supplies to the boys -- which could limit the time they can safely remain in the cave.
There are three main options on the table -- and each has its challenges.
One is bringing the boys out from the cave system before the rains come. Rescuers have been considering fitting them with full-face oxygen masks and accompanying them on a long, dangerous swim through the tunnels.
Another is finding a way to reach them from above. Teams are scouring the jungle-covered mountainsides in search of openings, or natural chimneys, that might lead to the small, dry, mud-covered incline where the boys and their coach remain perched above water.
The third is waiting until the flood waters subside after the monsoon season. But this could take months and rescuers are increasingly worried about oxygen levels in the cramped, pitch-black chamber and the tunnels leading to it, as well as the risk that rising waters overwhelm the ledge where the group are huddled.
More than 100 Navy Seals are now at the site in Chiang Rai, aided by international experts, as Thai authorities battle to bring all 13 out safely.
The loss of former Sgt. Saman Kunan, an ex-Seal who died while returning from delivering oxygen tanks to the cavern where the boys and their coach are located, has hammered home the difficulty of bringing the group out.
It takes even the most experienced divers up to five hours to swim through jagged, narrow channels from the chamber where the boys are to safety outside.
Even with full-face masks, the journey would be dangerous and arduous. The boys, who have been trapped inside the cave system since June 23, are in a weakened state, having spent nine days without food or fresh water before rescuers discovered them on July 2. Some cannot swim.
"Cave diving is incredibly dangerous for people who are very experienced doing it. And now you're looking at taking people who have no experience or very little experience with diving, and putting them into a complete blackout situation, where they have to rely on a regulator and the tanks with them to breathe," Anmar Mirza, a cave rescue expert and member of the US National Cave Rescue Commission, told CNN.
Songkran Yodpunkham, a 38-year-old diver with Thailand's electricity generating authority, has been pre-positioning oxygen tanks in the underground operation center, known as Chamber 3. He thinks the risks of swimming out are high but worth taking if there is no other option.
"The biggest issue is the difficulty facing anyone diving for the very first time. With these children, the danger is that they might panic," he told CNN.
"Their physical strength, their pulmonary function, and mental health have been under severe pressure. This could make them panic or cause them to choke and this would use up a lot of air. It could be terrible -- they could start hyperventilating.
"Many things could happen. It's zero visibility down there. You can't even see the face of your diving buddy. You can only do hand signals-- gestures like 'Are you still OK?'
"In the dark and in the water, it would be difficult for the divers to communicate with the children."
Finnish specialist diver Mikko Paasi, who's volunteering at the site, told CNN that news of Saman's death had hit hard but that everyone was still focused on keeping the boys alive and getting them out.
"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.
More heavy rainfall is expected to hit the area this weekend.
Thailand's monsoon season runs from July to October and, while the past few days have been relatively dry, the long-term forecast is rain for months.
"In the previous days we were fighting with time. And now we are working against water," Gov. Narongsak Osottanakorn told reporters Thursday.
Neil Bennett, the managing director and an instructor at New Zealand Diving, told CNN that the coming rain could undo all the work that's been done to drain the tunnels in the past few days.
"The mountain itself will work like a bathtub holding the water. The cave system below is effectively the escape route for that water. So as soon as the rain builds up in some part of the mountain system, the water level is going to rise in the caves," he said.
"The predictions of the water that's coming, of the rain that's coming, is going to heighten the danger now for those in the cave system. So they'll be back to a more difficult situation, with very strong flooding, very strong currents to fight against, and the visibility will be very poor."
If the water levels rise too far, the boys and their coach could drown in the chamber where they're waiting.
They already know well what even short-lived rainfall can do. They entered the Tham Luang Nang Non cave complex during fine weather but became trapped when a sudden downpour flooded the narrow tunnels.
Oxygen levels are down to around 15% in the chamber where the boys are, officials say, and can be depleted just by people breathing. At sea level, the normal oxygen level in the air we breathe is about 21%. At lower levels, the body struggles with exertion and thinking can become impaired.
"If they're in less than 16% or 15% oxygen, they're in a deficit already. That's no time to start diving," Bobby Chacon, a retired FBI special agent and former FBI dive team leader, told CNN.
Getting oxygen to the boys in the cave is the first priority, Thai Navy Seal Chief Rear Admiral Aphakorn Yoo-kongkaew said Friday.
Crews are working to install an oxygen pipe but it has only reached the underground operation center, two kilometers (1.2 miles) inside the cave, Yoo-kongkaew said. Water has now dropped down to levels where that chamber can be accessed by foot -- but from there, it is another two kilometers to reach the boys.
Yoo-kongkaew said the assessment of how long the boys can stay safely inside the cave had changed.
"We can no longer wait for all conditions (to be ready) because of the oppressive situation," he said at a news conference. "We originally thought the young boys could stay safe inside the cave for quite a long time but circumstances have changed. We have a limited amount of time."
Teams are scouring the rugged, tree-covered mountainsides above the cave system looking for an alternative entrance to reach the group trapped 800 meters or more below. But the difficult terrain and presence of many fissures, or chimneys, make the search slow going.
"In a big mountain system like this, the boys are in what we call the master cave, which is where all the water passage and everything leads, so there are lots of entrances that could eventually make it into the cave," said Josh Morris, founder of Thai climbing company Chiang Mai Rock Climbing Adventures.
Searchers are studying the geology for clues and looking for signs of cold air coming out through an opening, which could signal that it leads into the right part of the complex below, he said.
"The goal is to try and find the chimney, drill some bolts, put some ropes in and rappel down into different chimneys to wiggle down into the cave maze to see if we can get to the main passage," said Morris.
"The key is finding the right hole. If you find the right hole it may not take very long. It's just whether you can find the right hole because they are all over the hillside."
If a passage can be found, experts would consider this a safer way to extract the group than diving them out. But it would be too risky to try to drill down to them through the limestone, never mind the difficulties of getting heavy drilling equipment onto the mountainside.
Rescue teams have been pumping millions of liters of water from the cave in an attempt to lower water levels to the extent that the boys can simply walk out.
However, they could be fighting a losing battle if any more rain falls because of the porous rock type found inside the cave network.
"It's a giant sponge so if the water rises anywhere on the water table, it affects the whole cave system," said Tim Taylor, an experienced ocean explorer and expert on underwater robotics.
Yoo-Kongkaew said teams had found several springs in the cave that they blocked.
Paasi, the Finnish volunteer diver, told CNN that conditions have become less challenging because the water levels have dropped "quite a bit," thanks to the pumping efforts and lack of sustained rain. The current has also weakened, allowing divers to move more quickly, he said.
Nonetheless, visibility is still zero and the dive "is as technical as it can get," even for professionals, he said.
The psychological pressures are also tough. "If you lose it, you lose it," he said. "Keeping your head cool is the number one thing there."