The falling water level in the cave has helped the rescue effort, Chiang Rai's governor, Narongsak Osatanakorn, said on Saturday evening.
Medical teams staged drills on Saturday to prepare for their possible rescue as worries loomed over how the boys might be pulled out of the Tham Luang cave if and when they are found.
As the search entered its ninth day, rescuers still had no certainty of the whereabouts of the missing group, comprising a dozen boys aged 11-16 and their 25-year-old coach.
The desperate search has been beset by downpours that submerged tunnels near the entrance, but Thai Navy SEAL divers reached a T-junction in the cave's depths on Saturday - just two or three kilometres from where the boys are believed to be. This time divers left bottles of oxygen along the tunnel walls so swimmers navigating the muddy pools with nearly no visibility could stay under for longer.
"The situation is better today than yesterday and the day before".
The 12 boys and their coach entered the cave in Chiang Rai a week ago. "Hopefully we can start pumping out the well water by the end of today".
The group has little or no food and light, though they likely have access to fresh water - either dripping in through rocks or rushing in through the entrance - but experts warned that runoff water from nearby farms could carry unsafe chemicals or bacteria.
Even without food, he said, young, athletic boys could "easily live for a month or a month and a half".
Officials said tourists have been trapped in the cave and rescued in the past.
Rescuers have also been searching through jungle on the mountainside for alternative entrances to the caves.
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The natural chimney, which is around 1.5 meters in diameter and at least 22 meters (72 feet) deep, was found to the north of the opening of the cave, where the boys' bikes were found last Saturday.
Rescuers including Thai navy divers, a U.S. military team and British cave experts have been hampered by incessant rain that has flooded the cave.
But there was still no indication it linked to the main cave complex.
Australian, Chinese and Japanese experts also joined efforts.
"We have the skill to the rescue cave and the skill of SRT, single rope technology".
Large water pumps were also installed in a nearby village to drain water from the area.
Experts have said there is a good chance the limestone cave does have other entrances.
Stone-faced relatives kept vigil under a makeshift tent near shrines where monks are leading prayers near shrines overflowing with offerings.
Officials said the boys know the site well and have visited many times before, buoying hopes that they may have found an airy passage to seek shelter in.
Rescuers hope they made their way to an elevated rock mound in an underground chamber beneath a mountain - a spot they have nicknamed "Pattaya Beach".