Chiang Rai acting Gov. Narongsak Osattanakorn said the second phase began at 11 a.m. and authorities "hope to hear good news in the next few hours".
Rescuers working at a cave site in northern Thailand have suspended operations for the day after bringing four more boys out of the flooded cave system on Monday.
Musk said the aluminum shell of the so-called submarine had high thermal conductivity, meaning it is able to allow heat to pass through it easily, and therefore provide insulation to the person inside.
Candle-light vigils have been held in Thailand and parts of Asia, as the world watches on to find out the fate of the four remaining boys and their coach.
The eight Thai boys rescued from the caves over the weekend and Monday were quickly taken to the hospital and put in isolation rooms as medical teams evaluate the strain on their bodies from nearly two weeks underground.
Local officials refused to confirm the identities of those who had been saved, and their parents have not been informed of who is out.
But before the rescued boys can finally go home to their families, they need to make a pit stop at the hospital, where they're being briefly quarantined to make sure they didn't pick up any diseases in the caves, according to news reports.
Eight boys have now been rescued from Tham Luang Nang Non cave in Thailand after a youth soccer team intending to write their names on the wall of the tourist attraction was driven miles into the labyrinth by sudden monsoon rains that seasonally close and flood the caverns.
Operations kicked off at about 5am BST (11am Thai time) when a team of 18 elite Thai and worldwide divers entered the cave.
James Massola, on the ground in Chiang Rai province, covered the press conference after the rescue was wrapped up for the night, and reported that rescue mission chief Narongsak Osotanakorn said the team was "getting used to the operation". When the missing soccer team was located on July 2, rescue medics who reached them evaluated all the boys and their coach and divided them into three categories: red for critical condition, yellow for serious condition and green for stable condition. Wearing full-face masks, the boys either swam or were pulled along.
Foreign Minister Julie Bishop said lessons from the initial effort would be applied as two more groups of four were brought out of the cave.
Officials lavished praise on the Thai and global divers who, in pairs of two, executed the risky rescue mission, guiding the boys, who could barely swim and had no diving experience, through a treacherous 4-kilometer-long (2 1/2-mile) escape route that twisted and turned through the cavern. "If we bring five we have to change the plan". He had said fresh air tanks needed to be laid along the underwater route.
The death of military diver Saman Kunan on Friday underscored the huge risks the boys face.