Richard Stanton, one of a pair of British caving experts who located the "Wild Boars" team, gave reporters Friday a first-hand account of the moment he saw the boys emerge from behind a rock face onto a muddy ledge kilometres (miles) inside the Tham Luang cave.
"That was a massive, massive relief. Initially we weren't certain they were all alive -- as they were coming down I was counting them until I got to 13," he said.
"We couldn't see them initially -- they had to come round the corner."
The discovery prompted the stunning rescue of the boys which captivated Thailand and the world, with the final members finally emerging safely Tuesday after 18 days underground.
In order to rescue the boys, divers had to contend with a treacherous escape route made up of narrow, water-filled tunnels, with the threat of heavy rain injecting urgency to the bid.
The mission was "an order of difficulty much higher than anything that's been accomplished anywhere around the world by any other cave diving team," said Stanton.
Footage of the moment Stanton and John Volanthen discovered the 12 dishevelled and emaciated boys was viewed millions of times after it was posted on the official Facebook page of Thai Navy SEAL, prompting hope for their rescue.
"You hear on the video, John said 'How many?'," Stanton said. "I'd already counted them, they were already there."
Fellow diver Chris Jewell provided new details of the operation, describing how Thai authorities had diverted rivers on the mountaintop to help control water levels in the cave.
The measure "brought us additional time to get this outcome," Jewell said.
Stanton rejected suggestions the divers were heroes.
"We were just using a very, very unique skill set which we normally use for our own interest," he said. "Sometimes we are able to use that to give something back to the community and that's what we did."