Authorities on Sunday began the lengthy process of extracting a group of boys and their soccer coach from a flooded cave in northern Thailand, racing against impending rains to free them more than two weeks after the group was trapped there.

The boys and the coach will dive out of the cave one at a time, each accompanied by a member of a team of international and Thai divers, officials said. The first rescue is expected to be completed by 9 p.m. Sunday, though it could be two or three days before the whole group is freed.

"Today is the D-Day for the rescuers," said Narongsak Osottanakorn, the outgoing governor of Chiang Rai province and the lead official in the rescue. "We cannot wait any longer."

The water levels in the cave are the lowest they have been throughout this mission, authorities said, and the first few chambers that the boys will have to pass through — all of which were flooded days ago — are now dry. Oxygen levels, too, have stabilized following fears that the chamber the boys were in was filling with carbon dioxide from members of the large rescue operation.

Officials declined to say how far or how long the boys would have to dive after that point. They will be accompanied by an "all-star" team of 13 international and five Thai divers, officials said. A diver will be assigned to each of the boys and their 25-year-old coach, Ekapol Chanthawong.

"The kids are so strong, physically and mentally," Narongsak said. Officials have said that conditions are as perfect as they can be for the rescue attempt, factoring in the weather and water and oxygen levels in the cave, as well as the boys' health, which has been assessed in the cave by an Australian doctor. Heavy monsoon rains are about to fall over the lush mountain range that houses the cave, adding urgency to the rescue.

The young soccer players and their coach have been trapped in the vast cave system since June 23, when monsoon rains flooded the cave while they were exploring inside. The group was found alive in a small cave chamber on Monday after nine days, launching an international rescue effort of more than a thousand people, including divers from Australia, the U.K. and the U.S. military.

But experts have warned that extraction efforts will bear significant risk, underscored by the death of a retired Thai Navy SEAL early Friday morning when he ran out of oxygen during a dive. He was placing compressed air tanks along the boys' exit route when he fell unconscious and died shortly after.

Authorities for days had been stalling on a firm decision on the best path forward to extract the boys and their coach. Late Friday night, Narongsak admitted that all options appeared too risky and that officials were looking for backup options instead of the dive, spooked by the SEAL's death.

Authorities said efforts to drill down from the top of the mountain have been unsuccessful so far, with only 18 workable openings. The impending rains made that option less viable, as it will take too long.

Narongsak said that rescuers have assessed the boys' condition and also briefed their families, who are fully aware and supportive of the rescue plan. Officials have been racing against the clock to get the boys physically strong enough and mentally prepared for a journey that will probably take at least five hours.

"They are ready in every way," he said.

Before the rescue attempt was announced Sunday morning, ambulances were seen zipping up a muddy pathway to take their stations. Officials moved the large media contingent away from the rescue site to make way for those working directly on the extraction. They have not specified how the boys would be transported from the cave's entrance when they are freed, but several helicopters have been on standby for days, prepared to bring them to the hospital for treatment.

The drama of the rescue, with all its peaks of euphoria and lows of anxiety, has gripped the world, prompting experts from all over to weigh in on possible extraction methods that would minimize risk to the boys. SpaceX CEO Elon Musk said on Twitter that his team was working on a child-sized submarine pod that would be "small enough to get through narrow gaps." It was unclear whether his method would be ready in time or would be utilized by Thai officials.

Mental health and medical experts have warned that even if the whole team gets out safely, they are likely to battle weeks, if not months, of mental and physical traumas. Officials had said earlier that three in the group, including the coach, were weak and malnourished. Mental health effects could include depression, anxiety, anger and an inability to adjust to normal sleep patterns, said Jacob Hyde, an assistant professor of military psychology at the University of Denver who studies reactions to isolated, confined environments.

The group's camaraderie and Thailand's cultural context, however, would probably help them adjust after the ordeal, he said.

"Cultural factors have and will continue to come into play here," Hyde said. "If this were a group of boys who didn't know each other and were trapped in this cave, the process and outcomes would likely be quite a bit different."

The assistant coach with the boys in the cave, Ekapol, has been using his experience as a novice monk to help the kids stay calm through meditation, officials and parents have said.

The coach and the young boys have been communicating with their parents through letters, telling their loved ones not to worry and specifying their most pressing food cravings.

"If we can get out, please can you bring me to eat at the pan-fried pork restaurant?" one of the boys wrote in a letter published on the Thai Navy SEALs' Facebook page. "I love you."

In another post on their Facebook page, the Thai Navy SEALs said that their group was united in bringing out the Moo Pa, or Wild Boars, soccer academy, posting the message with a photo of three people gripping their wrists together.

"Hooyah," the post said in English.

Panaporn Wutwanich and Jittrapon Kaicome contributed.

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