The rescue got off to a good start, with the first boy coming out at 5.40pm local time (11.40am United Kingdom time).
Expert divers from Thailand, the United Kingdom and 16 other countries are now proceeding through the waterlogged passageways of the labyrinthine network to evacuate the remaining ten members of the Wild Boar soccer team and their coach in an operation dubbed "D-Day" by Chiang Rai Gov. Narongsak Osottanakorn, the head of the rescue mission. "We can not wait any longer".
Noting that a means to free the boys has not been decided, Mr Narongsak said that the water levels from the cave entrance to chamber three - where the rescue base is - is now low enough for personnel to walk through.
It will be an arduous operation, but the rescue teams are prepared.
Divers paired up to bring the boys out one by one, with officials warning the entire rescue effort could take days.
It's believed that two of the boys, as well as the 25-year-old coach, who were in a weak condition when discovered, have improved considerably since then.
"There's no time limit on the operation", said the governor.
But the Chiang Rai governor supervising the mission, Narongsak Osottanakorn, said earlier that mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation that won't last if it rains again.
It's part of an effort to evacuate the 12 boys from the team along with their coach, in a rescue that has captured the world's attention, with reporters flocking to the scene and foreign divers arriving to assist.
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Several of the boys emerged from the cave on Sunday, about eight hours after 18 divers began their operation to free them. The group was found alive in a small cave chamber on Monday after nine days, mounting an worldwide rescue effort with experts weighing in from all over the world. A former Thai Navy SEAL passed out making the dive on Friday and died.
The coach, Ekkapol Chantawong, who many Thais have criticized for leading the boys into the cave, also apologized. Officials were concerned that monsoon rains, which were forecast for today, would make such a rescue essentially impossible. The rescue was abandoned for the day, just after 8pm local (1am NZT). Impending rains made that option less viable, as it would take too long.
"They are ready in every way", he said. The ambulances drove to a nearby helipad and a helicopter was seen taking off. The rescued four boys are at a hospital near the cave. It was unclear whether his method would be ready in time or would be utilized by Thai officials.
Mr Narongsak said the boys and their coach, Ekkapol Chantawong are physically and mentally fit for the rescue bid.
"Four boys have reached chamber three and will walk out of the cave shortly", Lt. 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.
Officials had originally thought the group might have to stay where they were until the rainy season ended - that could have meant months underground.
"Cultural factors have and will continue to come into play here", Hyde said.
The boys and their coach are huddled together in a small chamber four kilometers (2.5 miles) inside the cave, surrounded by flood water and with a limited supply of oxygen.
The 12 members of a Thai soccer team trapped in a flooded cave network have written hopeful letters to their families on the outside, asking for fried chicken and BBQ when they escape.