Heavy rains that have battered the area are complicating the enormous rescue effort sending tonnes of water gushing through the complex.
Emergency workers say torrents have filled two chambers, blocking the route to the spot where the youngsters and their football coach might have retreated.
The darkness inside the Tham Luang cave in northern Chiang Rai province is being compounded by the muddy water, reducing visibility to just a few centimetres (inches), in conditions some rescuers have likened to swimming in cold coffee.
Thailand’s junta leader Prayut Chan-O-Cha arrived in the area early Friday to console desperate families who have camped outside the cave since the boys went missing on Saturday.
The harrowing search for the boys, aged 11 to 16, which was suspended for several hours Thursday was resumed when rescuers were able to drill a makeshift drain.
“They went in yesterday and they dived for several hours,” Chiang Rai governor Narongsak Osottanakorn told AFP.
Several water pumps were set up to get water away from the site and an additional 40 were being shipped in from Bangkok Friday, he added.
Other teams scoured the mountain for places to bore a hole through the cave’s roof without causing a collapse.
“It’s possible in theory but could be hard because the (drill) machine weighs two tonnes, so we have to figure out how to lift it and where to place it,” Narongsak said.
The British team found four potential entry points into the cave on Thursday, one of which was “promising”, the governor said.
The youngsters from the “Boar” football academy went in the cave after practice on Saturday and became trapped when heavy rains blocked the main entrance.
Search teams found their bicycles, backpacks and football boots near the opening.
Officials said the group has been in cave many times before and know the site well.
A sign at the entrance warns visitors not to enter the cave during rainy season between July and November, when the risk of flooding runs high.
At 10 kilometres (six miles) long, Tham Luang cave near the Laos and Myanmar borders is one of Thailand’s longest.
It is also one of the most difficult to navigate because of its narrow passageways and winding series of tunnels. —AFP