Now only four of the 12 youth football players are left inside the series of flooded chambers, along with their coach.
An ambulance with its emergency lights flashing has left the site of the underground Tham Luang Nang Non complex in Chiang Rai province where the youth football team has been trapped for two weeks. The ambulance was spotted leaving the site just hours after the second phase of a rescue operation was launched.
The vehicle drove towards a helipad, where a helicopter was seen taking off shortly afterwards to the cheers of the crowd below.
The same process was used for at least one of the four boys rescued in the first attempt at getting the 12 boys and their coach out of the cave.
Acting governor Narongsak Osatanakorn said authorities "hope to hear good news in the next few hours".
A total of five people remained trapped in the cave, including the team's coach, as of Monday afternoon.
"All conditions are still as good as they were yesterday," Mr Narongsak told a news conference.
"The boys' strength, the plan - today we are ready like before. And we will do it faster, because we are afraid of the rain."
Authorities have been rushing to extract the boys, aged 11-16, and their coach from the cave as the annual monsoon bears down on the mountainous region in far northern province.
Workers have been labouring round the clock to pump water out of the cave, and authorities said heavy downpours overnight did not raise water levels inside.
The four boys pulled from the cave on Sunday in a dangerous operation that involved them diving through the cave's dark, tight and twisting passages were happy and in good health, authorities said.
"This morning they said they were hungry and wanted to eat khao pad grapao," Mr Narongsak said, referring to a Thai dish of meat fried with chili and basil and served over rice.
Still, the four were undergoing medical checks in hospital in the provincial capital and were not yet allowed close contact with relatives due to fear of infections.
Relatives were able to see them through a glass partition, the governor said.
The boys and their coach went exploring in the massive Tham Luang Nang Non cave on June 23 after football practice, and were cut off when a rainstorm flooded the cave.
A massive international search operation was launched and it took 10 days to locate the boys, who had taken shelter on a dry slope deep in the complex.
The search and rescue operation has riveted people both in Thailand and internationally, with journalists from across the globe traveling to this town along the border with Myanmar to report on the ordeal.
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 more quickly.
Sunday's mission involved 13 foreign divers and five Thai navy Seals. Two divers accompanied each of the boys, all of whom have been learning to dive only since July 2, when searchers found them.
The death on Friday of a former Thai navy Seal underscored the risks. The diver, the first fatality of the rescue effort, was working in a volunteer capacity and died on a mission to place air canisters along the passage to where the boys are, necessary for divers to safely travel the five- to six-hour route.