Authorities said at a press conference Sunday morning in Chiang Rai province that they made the decision to rescue the boys as oxygen drops and the threat of monsoon rains approaches. Due to the length of the journey out of the cave, officials said the first boy was expected to come out at 9 p.m. local time, which is 10 a.m. Sunday Eastern time. The officials said the operation could take two or three days.
At 10 a.m. local time, 13 foreign divers and five Thai SEALs entered the cave to begin the operation. Two divers will escort each of the kids out of the cave.
“We have a fraction of a second to help them come out,” provincial Gov. Narongsak Osatanakorn said.
It will take five hours for rescuers to reach the boys from the entrance of the cave, and six hours to bring back the boys, including an hour break time.
“They insist they are ready to come out,” Narongsak said Sunday of the rescue team. “Family has already been notified.”
The most challenging part of the journey for the boys will be the first leg of the trip.
Once they make it to the third chamber, they will have one hour of rest. Then the rest of the journey will be on foot walking through muddy areas or floating in some areas.
“There are two obstacles, which are water and time,” Narongsak said. “We have tried all possible ways to get the boys out, but sometimes we can’t win over nature.”
Water levels were down 30 percent, an official said at the press conference — the lowest it’s been since the boys were located.
The officials also said they had ruled out the potential for drilling into the cave from above.
The boys are well aware of the mission involved, and the parents also understand what’s at stake, according to Narongsak.
“Experts’ assessments indicate that today is the most ready we can be,” said Choorat Panngao, Provincial Police Region 5 deputy commander. “If we don’t do it today we will lose our opportunity.”
Seasonal monsoon rains forecast to hit the region this weekend and throughout next week have yet to occur, and efforts on the ground to remove floodwater and divert water flows have been “very successful,” Osatanakorn said Saturday. Rescuers can now walk, rather than swim or dive, from the cave’s main entrance to an inner chamber serving as a command center.
“We are very happy with the water situation here,” he said. “The perfect situation would be to have zero water, which is impossible. The water level would be zero during December or January, so this situation is absolutely impossible. The next best situation would be if the water level is as low as possible to move the kids.”
Despite the progress with diverting water, Narongsak said on Saturday that rescuers are struggling to maintain safe oxygen levels inside the cave.
The inner chamber where rescuers have set up a “forward operating base” now has low oxygen levels due to the amount of people inside. They have tried pumping “pure air” through a tube into the chamber, but have also had to pull back nonessential personnel in an effort to preserve oxygen, according to Narongsak.
Officials said they were drilling all day Saturday to transport the children to the hospital once brought out of the cave.
“We are preparing for air evacuations in two different places,” Choorat said. “Additionally we have 13 ambulances standing by.”
ABC News’ Rex Sakamoto contributed to this report.