Authorities in northern Chiang Rai province began the risky mission to bring out the 12 boys and their coach earlier on Sunday.
An ambulance carries one of the boys rescued from Tham Luang Nang Non cave in Thailand.
Volunteers outside the cave in Chiang Rai cheered as news of the first boy rescued on Monday emerged.
Officials said that rescue operations could continue for two to four more days. The search and rescue operation has involved dozens of worldwide experts and rescuers, including a USA military team.
"I would like to inform the public at home and those who have been giving us support all along, after 16 days, today's the day we've been waiting for, we are seeing the Wild Boars in the flesh now".
Authorities on Monday confirmed that water inflows into the cave remained at controllable levels while drainage efforts continued.
A spokesman for Musk's Boring Co. tunneling unit, which has four engineers at the cave, has said Thai officials requested the device, which could potentially help the children through narrow, flooded cave passageways. The scheduled time was advanced from Monday night to race against time.
"When everything was taken into account, including a forecast of possible rain, the water level and the physical health of those involved, the operation was advanced 4-5 hours".
The operation to rescue the Thai football team trapped in a cave has begun this morning.
"We have more operating personnel". But she added she was happy at the prospect of seeing her son again. Still, they were undergoing medical checks in a hospital and were not yet allowed close contact with relatives due to fear of infections, who were able to see them through a glass partition. He said rescuers told the parents that the "strongest children" would be brought out first.
Sunday's "masterpiece" three-and-a-half-hour mission saw four children being calmly guided to safety after 15 days of being stuck in their fetid underground prison.
When Sgt. -Maj. Saman Gunan, a former Thai navy Seal, died while laying oxygen tanks along the route out of the cave, people grieved and became fearful about what could lie ahead for the team. Moreover, not all the boys know how to swim, let alone dive underwater.
Lastly came Pipat Bodhu, 15, aka Nick, who was not even in the team but came along as a friend of the goalkeeper.
The toughest part of the route is a section at nearly halfway called "T-Junction", which due to its narrowness causes divers to remove their air tanks to fit through.
The names of the rescued boys have not been released out of respect for the families whose sons were still inside, and they have not been reunited with their own families, he added.
Boys from the soccer team trapped inside Tham Luang cave covered in hypothermia blankets react to the camera in Chiang Rai, Thailand, in this still image taken from a July 3, 2018 video by Thai Navy Seal.