Rescuers are racing against the rains to free 12 boys and their football coach, who are trapped in a flooded cave in northern Thailand.
A deluge is expected to hit in a matter of days that could force the water level up, threatening to flood the pocket where the group took refuge.
The teenage boys and their 25-year-old coach have been trapped for 12 days.
They were found on Monday night by rescue divers, on a rock shelf about 4km (2.5 miles) from the cave mouth.
The boys have now received food, foil blankets, and medical attention, and rescuers are trying to run cables through the cave tunnel so they can speak to their families.
When are the rains coming?
The region of Chiang Rai where the boys are trapped has for the past few days experienced a dry spell, and rescuers have taken advantage of this to pump water out of the cave complex.
About 128 million litres of water had been pumped out by Thursday, with the water levels coming down at an average rate of 1.5cm per hour. Rescue workers are now able to walk through a 1.5km (0.9 mile) stretch from the entrance to what's being called the third chamber.
But heavy monsoon rains are forecast for Sunday. Chiang Rai Governor Narongsak Osotthanakorn said they were "racing against water".
"We are calculating how much time we have it if rains, how many hours and days," he said.
The Tham Luang cave complex is regularly flooded during the rainy season until September or October, raising fears that a delay could leave the boys trapped in the cave for months.
What is being done to rescue the boys?
There is hope that enough water can be pumped out of the cave tunnels for the boys to be able to wade - or be floated - out.
But rescuers are also planning for other eventualities.
A team are exploring the forested mountain land above the cave complex to see if they can find a chimney down to the cavern sheltering the boys. They have enlisted the help of bird-watchers, who are specialists in finding hidden holes, the AFP news agency reports.
Thai Navy Seal divers are also teaching the boys the basics of diving, with a view to guiding them out through flooded waters if necessary.
But such a rescue would be fraught with risk, say experts. Many of the boys cannot swim or dive, and there is a high risk they might panic in the dark, murky, narrow waterways.
The journey for the group to travel up to the cave entrance would take around five hours, rescue divers say.
The Thai military has previously said that if the boys can't dive out, the group may have to wait for up to four months for flooding to recede before they can leave.
Food and other supplies are being put in place for that eventuality.
'The boys are my brothers'
Helier Cheung, BBC News, Tham Luang cave
Rescue teams are working in extremely difficult conditions. The heat has been sweltering and unrelenting, at over 30C, while much of the site is submerged in squelchy mud that is several inches thick in places and extremely slippery.
But the work has continued at an unrelenting pace. Everyone is aware that once the monsoon rains start, rescue efforts will be much more difficult.
Meanwhile, hundreds of workers are helping to keep the site in order, with road engineers laying gravel to try and secure the mud, workers for the Thai royal kitchen providing hot food for everyone, and volunteers handing out water bottles and ice lollies to those on site.
One local civil servant, who had volunteered to help hand out supplies, said he did not know the boys personally but had decided to help because "I consider the boys in the caves as my brothers".
How are the boys coping?
The arrival of food, foil blankets and the prospect of speaking to their loved ones will have buoyed their spirits.
But concern is mounting for both their physical and mental health after 12 days below ground - and it is not clear how they would be helped to survive months more in the cave.
The boys and their coach had gone on their bikes up to the caves on Saturday 23 June after football practice. It was one boy's 16th birthday, and the team had taken a picnic.
They knew the caves, and some reports suggest they had wanted to explore deeper than on previous occasions.
They entered the cave when it was dry but sudden heavy rains quickly flooded the exit and rushed through the narrow passages, clogging them with mud and debris.