More than 100 exploratory holes have been bored — some shallow, but the longest 400 metres deep — into the mountainside in an attempt to open a second evacuation route and avoid forcing the boys into a dangerous dive through submerged tunnels.

Conditions are “perfect” to evacuate a young football team from a flooded Thai cave in the coming days before fresh rains and a possible rise in carbon dioxide further imperil the group, the rescue mission chief said on July 7.

The plight of 12 Thai boys and their coach from the “Wild Boar” football team has transfixed Thailand since they became trapped in a cramped chamber of the Tham Luang cave complex on June 23.

Rescuers have conceded that evacuating the boys is a race against time with monsoon rains expected to undo days of round-the-clock drainage of the deluged cave.

More than 100 exploratory holes have been bored — some shallow, but the longest 400 metres deep — into the mountainside in an attempt to open a second evacuation route and avoid forcing the boys into a dangerous dive through submerged tunnels.

“Now and in the next three or four days, the conditions are perfect (for evacuation) in terms of the water, the weather and the boys' health,” Narongsak Osottanakorn, the Governor of Chiang Rai Province and the chief of the rescue operation told reporters. “We have to make a clear decision on what we can do.”

Rescuers have fed a kilometres-long air pipe into the cave to restore oxygen levels in the chamber where the team are sheltering, accompanied by medics and expert divers.

“When we're in a confined space if the oxygen drops to 12 percent the human body starts to slow down and people can fall unconscious,” Mr. Narongsak said. “There's also carbon dioxide. If the oxygen levels are down and the carbon dioxide levels are up, then you can get too much carbon dioxide in your blood.”

Heavy rains could make the water rise to the shelf where the children are sitting, reducing the area to “less than 10 square metres,” he added, citing estimates from cave divers and experts.

In the early hours of July 7, he said the boys were not yet ready to dive-out of the cave, a complex and dangerous task through twisting and jagged submerged passageways. But his comments 12 hours later suggest the thinking has changed, with water levels inside the cave currently managed to their lowest point by constant drainage.

‘I love you Dad, Mum’

Earlier in the day, Thai Navy SEALS published touching notes scrawled by the trapped footballers to their families, who are waiting for them agonisingly close-by outside the cave entrance. The children urged relatives “not to worry” and asked for their favourite food once they are safely evacuated.