The boys – some are non-swimmers and all are brand-new learner divers – will make their perilous escape through narrow tunnels that are at one point just 15 inches wide, challenging even the world’s leading cave divers.
Rescuers have begun a mission to evacuate 12 boys and their football coach from the cave where they have been trapped for two weeks.
Teams entered the cave at 10:00 local time (03:00 GMT), operation chief Narongsak Osottanakorn told reporters.
All non-essential staff have been cleared from the site near the entrance, leaving only diving teams, medics and security forces.
The boys were exploring the cave on 23 June when they were cut off by water.
Thirteen foreign divers and five from the Thai Navy are involved in the rescue mission.
The earliest the first boys could emerge is 21:00 local time (14:00 GMT) on Sunday, according to officials. It is thought the whole operation could last several days.
Both the trapped group and their families had been informed of the plan.
Mr Narongsak said that the boys and their coach were “very fit physically and mentally. They are determined and focused.”
He added: “This is D-Day.”
It is unclear exactly how the boys will be moved but it appears that the journey will include some diving and walking in areas where the water level permits.