Ekapol Chantawong has been trapped in the Tham Luang cave with 12 boys from the football team he coaches since 23 June.
The 25-year-old was the only adult in the group.
As of yesterday, 9 July, eight of the 13 trapped in the cave have been rescued and are reportedly in good health.
Thai authorities confirmed that the third phase of the rescue mission began at 10.08am today, 10 July, with 19 divers on their way to rescue the four boys and the coach still inside the cave.
Rescue officials confirmed that when the group was discovered after a painstaking 10-day long search, Ekapol was one of the weakest among them
The boys told rescuers that their coach taught them how to meditate to preserve their energy while they waited for help
Ekapol, who lost his parents at 10 years old, trained as a Buddhist monk at a monastery in Mae Said, Thailand for a decade since he was 12 years old. Three years ago, he left the monastery to care for his sick grandmother and help train the newly established Moo Pa team.
The 25-year-old's aunt said that, "He could meditate for up to an hour. It has definitely helped him and probably helps the boys to stay calm."
Before the rescue operations began, the group wrote letters to be delivered to their loved ones.
Ekapol sought forgiveness from the boys' parents in a letter addressed to them.
"I promise I will care for the kids as best as possible. I want to say thank you for all the support and I want to apologise to the parents," he wrote, according to Fox News.
In a joint letter to the coach, the parents told the 25-year-old not to blame himself. "We want you to be relieved. No father or mother is angry with you at all. Everyone understands and wants to encourage you," it read.
"If he didn't go with them, what would have happened to my child?" said the mother of Pornchai Khamluang, one of the boys in the cave, in an interview with a Thai television network.
"When he comes out, we have to heal his heart," she added.
"To survive, they are all heroes, but the biggest hero is the coach. I'm sure he has done everything for all the kids in the cave," said Ekapol's friend, Auttaporn Khamheng, The Sydney Morning Herald reported.
The boys wrote encouraging notes to their families to keep their spirits high ahead of the rescue operations: