Authorities had earlier Sunday told the more than 1,000 journalists who had converged near the cave site's entrance to clear the area in preparation for the rescue operation.
On Sunday morning, Governor Narongsak Osottanakorn said a team of 13 global divers and five Thai Navy SEALs went into the cave to commence the mission.
Media have been asked to leave the area by 9am local time (noon, AEST) Sunday, and the site was evacuated for a "rescue operation".
According to the BBC, the highly trained diver ran out of of oxygen during the 11 hour trip in and out of the cave and was unable to be revived by his diving partner.
The order to leave the site came a day after the rescue mission chief said conditions were ideal for the evacuation to begin, but also as fears mounted that expected rains could thwart the plan by reflooding the cave.
Rescuers in Thailand have kicked off an operation to pull 12 members of a junior football team and their coach from a flooded cave that they've been in for over two weeks, the head of the rescue mission has announced.
The handwritten letters from the group were handed to British divers on Friday and released on the Thai Navy Seal Facebook page on Saturday.
"Don't worry, I'm fine".
"We want you to know that no parents are angry with you at all, so don't you worry about that".
The rescue began as dark monsoon rainclouds loomed over the mountainous north of the country. Tell my brother to be ready to bring me fried chicken.
"Night loves Dad and Mom and brother, don't worry about me", wrote another boy in the Thai manner of referring to one's self in the third person.
Another joked: "Teacher, don't give us lots of homework!"
The coach of a youth football team trapped for two weeks in a flooded Thai cave has sent his "apologies" to their parents in a scrawled note handed to divers, as officials Saturday appeared to rule out an immediate rescue.
"We did an assessment and we think it is quite unsafe at this point to bring them out".
"To the parents of all the kids, right now the kids are all fine".
Others have criticised him for agreeing to take the young boys into the cave during the monsoon season. "I promise to take the very best care of the kids", he said in a note given to divers on Friday. "I love you", wrote 11-year-old Chanintr Wiboonroongruang (Titan).
While the oxygen level had stabilised he warned levels of carbon dioxide in the cave were another factor in considering when to move the group, in addition to impending rains, which could cover much of the muddy ledge on which the group are sheltering.
Authorities are also looking at alternative options like finding a shaft or drilling into the mountain to create the equivalent of a "back door" into the cave.