The acting Chiang Rai governor told reporters "today is D-Day" with 13 foreigners and five Thai divers taking part in the rescue. He says the divers went in at 10 a.m. local time on Sunday and the boys will gradually come out accompanied by two divers each.
It could be hours before anyone emerges, a Thai army commander said, noting the rescue could take two to four days depending on conditions inside the partially flooded cave.
Thai authorities are calling it a war against the water. Divers are working around the clock, trying to rescue the boys before heavy rains hit again.
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Each of the boys will go on what's being called a buddy dive, essentially hauled by two divers through tunnels sometimes only as wide as a manhole cover.
Over the past several days divers have been teaching the boys how to dive, and some are learning the basics of swimming for the very first time.
Thai Navy Seals posted this image to Facebook after the rescue operation was announced.
As the world waits for their escape, the boys have written letters to their parents. Some asked for chicken and barbecue pork. Others asked that they not get too much homework when they're free.
The boys' coach also wrote a letter. "I promise I will take care of the kids," he said. "Thank you for your kind support and I would like to apologize to the parents."
The boys, ages 11 to 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.
ABC7 News contributed to this story.