After the ambulance was seen leaving the complex at around 5 p.m. Monday, a helicopter took off.
A source at the hospital said five emergency response doctors were waiting for the party and a further 30 doctors were on stand-by, adding that everyone was feeling tense.
The governor explained that a re-evaluation of the boys' situation had led the rescue authorities to conclude that they could not stay in the cave throughout the monsoon season.
Chiang Rai Governor Narongsak Osottanakorm recently said the next phase of the rescue operation would start on Monday morning.
"Today is D-Day", Narongsak had earlier told reporters.
Four boys from a Thai football team have been rescued after 15 days trapped deep within a cave system in Chiang Rai - leaving behind eight boys and their coach as the rescue operation continues.
SBS News understands that the boys are in a sterile, isolated section of Chiang Rai's provincial hospital.
The second evacuation attempt started at 11 a.m. local time (midnight ET) after rescue workers got some rest and refilled supplies. It takes several hours.
They were followed shortly afterwards by four others, leading to an explosion of jubilation on social media in Thailand and around the world as the rescued boys were rushed to hospital.
Officials told media gathered at the site that the boys and their coach are in strong spirits and determined to get out. The operation is set to resume later.
The official heading the operation to rescue 12 boys and their soccer coach from a cave in northern Thailand says the operation is going "better than expected".
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Elite divers on Sunday began the extremely unsafe operation to extract 12 boys and their football coach who have been trapped in a flooded cave complex in northern Thailand for more than two weeks, with some of the rescuers reaching the area where the team is sheltering. Rescue teams have made the pause to replace the air tanks.
According to Thailand's interior minister, same divers who rescued 3 students and the coach on Sunday night will conduct the next operation as they are aware of the cave conditions. Officials did not immediately comment.
Two divers are helping each of the boys out as families wait nervously outside the cave entrance - 2.5 miles (4km) from where they were trapped.
But multiple forecasts predict a chance of more rain overnight and the threat of thunderstorms in coming days.
The next operation will begin in 10 to 20 hours and will involve 90 divers, he said.
Before the current rescue mission began, Elon Musk tweeted that he was designing an "escape pod" built from "the liquid oxygen transfer tube" of a SpaceX Falcon rocket that could be used as a submarine.
Videos showed Musk's team testing the metallic pod in an LA-area high school swimming pool.
On Sunday four members of the "Wild Boar" team were successfully brought out from the cave, after authorities decided they had to rush ahead with a rescue operation to beat monsoon rains.
A spokesman for Boring Co. said on Sunday that the company has four engineers who are "offering support in any way the government deems useful".
Authorities have said extracting the entire team from the cave could take up to four days, but Sunday's success raised hopes that it could be done faster.
Authorities in northern Chiang Rai province began the risky mission to bring out the 12 boys and their coach earlier on Sunday. All four were rushed to a hospital 65km away in Chiang Rai. The search and rescue operation has involved dozens of global experts and rescuers, including a USA military team.
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