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.
Anticipation in the air
By Helier Cheung, Tham Luang cave
This is the moment that everyone has been waiting for - and that some thought would never begin.
The first hint that a rescue could be imminent came when all non-essential volunteers and media were asked to leave the site of the caves.
The relatives of the boys were also moved from the rooms where they had been staying.
Earlier on, the mood around the rescue sites had been upbeat once the boys were found alive. Smiling volunteers would hand out snacks, or offer massages, to those on the site.
Now security to the site has been beefed up even more. Several police vans, military officers and ambulances have been entering and exiting the site. There's a solemn sense of anticipation in the air.
Family and friends of the boys will be getting little rest until they learn the outcome of the rescue operation.
The initial part of their journey, through narrow, submerged passages, is the most challenging, the BBC's Jonathan Head reports from near the cave.
It could involve long periods underwater for children as young as 11 who have never used diving equipment before.
In an indication of quite how dangerous the journey will be, a former Thai Navy diver died in the caves earlier this week. He was returning from a mission to provide the group with air tanks.
The rescuers plan to rest them in a cave, which has been the forward base for the divers, our correspondent says.
They will then make the last, easier walk out to the entrance, and go from there to hospital.
The Thai Navy Seals posted an image after the rescue operation was announced, showing rescue workers holding each others' arms.
