Authorities have highlighted the tiny passageway near T-junction, or Sam Yak in Thai, as the most unsafe element of the journey for the "Wild Boars" team that began on Sunday morning, but there are many other potential pitfalls.
Five Thai and 13 foreign divers are involved in the mission, which will see each of the boys escorted by two divers through the cave's dark and twisting flooded passageways.
The operation began at 10 a.m. Shortly before 8 p.m., the SEALs reported on their official Facebook page that four had been rescued.
That's the name of the young boys' team. An Australian doctor who is part of Sunday's rescue mission checked the health of the boys on Saturday night and gave the all clear for the rescue to proceed.
Narongsak said Sunday two divers would accompany each of the boys out of the cave.
Experienced divers take approximately 11 hours to complete the entire round trip so the first boys could be expected to emerge some time after 9pm local time (2am NZ time).
"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".
Flooding cut off their escape and prevented rescuers from finding them for nearly 10 days.
The statement added: "due to the complex nature of the cave and the difficulty of the operation we can not yet verify how many children are coming out in this first operation". "(There) might be crucial changes in the rescue operation (plan) and somebody will make a decision maybe to bring more of them out today".
The notes were sent out with divers who made an 11-hour, back-and-forth journey to act as postmen.
A former member of Thailand's SEAL unit died during a dive early on Friday morning.
Ekkapol's message is the first from the coach, whose role in the team's predicament has split Thai social media.
An "all-star" team of 18 divers entered the caves this afternoon as the boys and their families were notified of the development.
A Thai army commander said the ongoing rescue could take 2-4 days.
Gong Hui, a Chinese diver involved in the operation that has drawn some 130 Thai and global divers, told Reuters on Saturday before the fresh rains that water levels in the cave had "receded a lot" after sustained pumping had removed millions of litres of water.
Mission chief Narongsak said in recent days that medic teams had complained about the media presence and they told him "it will be a problem if they have a real emergency situation". An incredible collective effort is on display in a makeshift village while authorities offer emotional support for the parents and families who are in tents desperately waiting for their boys to come out of the cave after almost two weeks.
Of the 13-strong foreign dive team - mainly from Europe - three will escort the children, while the remainder are positioned along the risky first kilometre stretch, where the boys will have to navigate through submerged passageways in some places no more than two feet (0.6 metre) wide.
In another development on Saturday, authorities confirmed the boys had communicated by letter with their relatives, many camped outside the cave's entrance, for the first time since they were found.
The only way to bring them out is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air. It is considered very hard even for expert divers to navigate the escape route.
When facing particularly narrow paths, the divers will release the tanks from their backs and slowly roll it as they guide the boy through the passage.
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