The acting Chiang Rai governor has told reporters "today is D-Day'' with 13 foreigner and five Thai divers taking part in the rescue.
He says the divers went in at 10 a.m. and the boys will gradually come out accompanied by two divers each. He says the earliest they will come out is 9 p.m. Sunday (10 a.m. ET).
The only way to bring them out is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.
Experienced cave rescue experts consider an underwater escape a last resort, especially with people untrained in diving, as the boys are. The path out is considered especially complicated because of twists and turns in narrow flooded passages.
The boys, ages 11-16, and their 25-year-old coach became stranded when they went exploring in the cave after a practice game. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.
The only way to reach them was by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.
The path out is considered especially complicated because of twists and turns in narrow flooded passages.
The boys sounded calm and reassuring in handwritten notes to their families that were made public Saturday. The notes were sent out with divers who made an 11-hour, back-and-forth journey to act as postmen.
One of the boys, identified as Tun, wrote: "Mom and Dad, please don't worry, I am fine. I've told Yod to get ready to take me out for fried chicken. With love."
"Don't be worried, I miss everyone. Grandpa, Uncle, Mom, Dad and siblings, I love you all. I'm happy being here inside, the navy SEALS have taken good care. Love you all," Mick wrote.
"Night loves Dad and Mom and brother, don't worry about me. Night loves you all,'' wrote Night, in the Thai manner of referring to one's self in the third person.
The most touching note came from one whose name was not clear: "I'm doing fine, but the air is a little cold, but don't worry. Although, don't forget to set up my birthday party.''
Another, of indistinct origin, asked their teacher not to give them a lot of homework.
In a letter of his own, the coach, Ekapol Chanthawong, apologized to the boys' parents for the ordeal.
"To the parents of all the kids, right now the kids are all fine, the crew are taking good care. I promise I will care for the kids as best as possible. I want to say thanks for all the support and I want to apologize to the parents," he wrote.
An update Saturday from the Thai navy said three navy SEALs, one a doctor, were with the boys and their coach. The 13 are having health evaluations and rehabilitation and are being taught diving skills. Food, electrolyte drinks, drinking water, medicine and oxygen canisters have been delivered to them. A major concern of the rescuers is that oxygen levels in their safe space could fall dangerously low.
Rescuers have been unable to extend a hose pumping oxygen all the way to where the boys are, but they have brought them some oxygen tanks.
The death on Friday of a former Thai navy SEAL, Saman Gunan, underscored the risks of making the underwater journey. The diver, the first fatality of the rescue effort, was working in a volunteer capacity and died on a mission to place oxygen canisters along the route to where the boys and others are sheltered.
Information from The Associated Press was used in this report.