The head of the operation, Narongsak Osottanakorn, said earlier the final operation would be "more challenging" because one more survivor would be brought out, along with three Navy SEALs who have been accompanying them.
So far the group have been "kept away" from physical contact with their parents until the risk of infection is over and doctors have only allowed family and loved ones to see them through glass.
Officials at the Chiang Rai hospital said on Tuesday morning that eight of the boys that have been rescued already are in good health and are recovering in the hospital.
The CEO of Tesla and SpaceX was - if his Twitter and Instagram accounts are to be believed - in Thailand on Monday touring the cave where a team of 12 soccer players, aged 11 to 16, became stuck with their coach on June 23. "We can see that everything is ok as they're eating well", he said. It was clear doctors were taking a cautious approach.
The first four boys who were rescued from a Thai cave on Sunday have been given some chocolate.
MAE SAI, Thailand-Thai authorities hope to evacuate four remaining boys and their soccer coach from the flooded Tham Luang cave on Tuesday, ending a more than two-week ordeal that began when members of the team were stranded by flash floods deep inside one of the country's biggest cave systems.
Four ambulances and a convoy of other vehicles arrived at the cave site Tuesday morning as rains hit the region.
Musk also said that a second sub, which is apparently 30cm shorter than the first one designed, was nearly complete, meaning that more than one device would be available for the rescues.
He said he hoped the rescue mission, which would begin with 19 divers and include more throughout the day, would proceed even faster than Monday's nine-hour operation.
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Jesada Chokedamrongsuk, a physician from the Thai ministry of public health, told a separate press conference at the Chiang Rai hospital that the eight patients were cheerful.
Thai Navy SEAL divers enter the cave searching for the boys.
Authorities then struggled to devise a safe plan to get them out, mulling ideas such as drilling holes into the mountain or waiting months until monsoon rains ended and they could walk out.
Dr. Andrew Adesman, chief of developmental pediatrics at Cohen Children's Medical Center in New Hyde Park, New York, said there is a threat of post-traumatic stress disorder (PTSD) for the boys and their coach, but he said he is hopeful it won't be an issue for a majority of the kids.
Because of the heavy rains, which eventually eased in the afternoon, Narongsak said the decision had been taken to "not to waste time ... for the third planned mission".
"If I ask too much, he might not provide it".
Officials described the rescued boys as "hungry but happy". Their friends were full of optimism - and worry.
Four boys and their 25-year-old football coach "will be extracted on Tuesday". "Even when my friends have left the cave, I'm anxious about their physical wellbeing".
The diver said that the fate of the boys is weighing on the teams' minds, and that they can't help but think of their own children.
Divers lead four of the boys out of the cave as night falls, sending them to the hospital.