Trekkers discovered the potential access point in the Pha Mee hills above the Tham Luang Nang Non Cave system, and are now exploring the possibility of their being linked to the caves below where the group has been trapped for nearly a week.
The natural chimney, which is about 1.5 meters in diameter and at least 22 meters deep, was found north of the mouth of the cave where the boy's bikes were found last Saturday.
Approach The tunnels from the top are "more promising" at the scene, according to British rescuers, as they bypass a passage blocked by the flood.
Two British cave experts broke into the chimney on Friday morning and managed to sink to a depth of about 20 meters. Another climbing team is now entering to relieve them. Although this is not the first natural chimney to be found on Pha Mee, it remains the most promising spot, according to local search and rescue teams.
Thermal imaging drones are also used to search the hilly terrain above the cave system.
Previously, trekking teams found a vertical shaft that seemed to lead into the cave, causing food and drinking water to be spilled if the group was nearby, said Thanya Netithamkul, director-general of the National Park.
The terrain is tough – mountainous and heavily wooded, slowing down the teams looking for other possible entry points, and the rains in that region were "relentless," according to CNN meteorologist Derek Van Dam.
"It's the southwest monsoon and the rains continue," he says, further complicating the rescue effort.
In addition to the British cave experts who were brought into the search area, a US search and rescue team has joined the rescue, at the request of the Thai government, said Pentagon spokesman Lt. Col. Dave Eastburn.
"The United States Team Delivers Search and Rescue Experience and Capacities to the Already Huge Efforts of the Thai Authorities I hope the players and their coaches get home quickly and safely."
The boys at the age of 11 to 16 years old and her 25-year-old coach missing since Saturday afternoon when a park officer spotted bicycles at the entrance
Thai Prime Minister Prayut Chan-o-cha visited the rescue center on Friday and met with officials and families the boy. He urged that rescuers should proceed quickly but with caution, particularly with regard to the installation of electrical systems to allow water pumps to operate efficiently.
It is believed that the boys and their coach crawled through a narrow, 15-meter-long canal into the great cave.
A sign at the entrance The cave – a popular tourist attraction – warns of the danger during the rainy season, which is just beginning.
The rescue teams are currently maintaining two theories about where the boys might be by working from the starting point of their bags, at a fork 3 kilometers from the mouth of the cave, deep into the underground tunnels.
At that point, they could have turned left or right, but the Thai rescuers assumed they took the next higher point to the left – south.
British experts are now wondering if they have gone to the right, taking them to the highest point – north, where the chimney is being explored.
Divers continue to work at the mouth of the cave, and drilling is being carried out at several points of the cave complex (19659002) Pumps are also used to remove the water from cave tunnels rather than providing access.
Josh Morris, founder of Chiang Mai's Rock Climbing Adventures, who is familiar with the terrain, says that despite the ticking clock, there is reason to be optimistic about the survival of the group.
"I'm sure there's still hope.The boys are still in the first week and the boys who went into that cave from wh I understand I've been in the cave many times, so it's very likely they somehow knew where they were going and were able to move to a safe place. "He told CNN Connect the World.
"The problem is because of the heavy rains that hit the northern hills of Thailand in the last two weeks, there are only increasing amounts of water that continue to grow, which is really the biggest challenge now."
Dozens of relatives of the boys have gathered at the entrance to the cave, and many have been there since Saturday night, just after they knew of their sons' disappearance.
Some have collapsed from exhaustion and had to be brought near hospitals. Others have found support in group rituals, praying and giving gifts of food and flowers to ghosts who are believed to inhabit the cave.
Even as the days wear out, some of the family members remain convinced that they are being found safe and sound.
Sudsakorn Sutham, the father of a missing boy, Prachak, said he was sure the authorities would bring the boys home.
"I feel (the situation) is getting better, I'm confident that my son will come back," he said. "There's so much help here."
He said he believed that Sporting his son will help him survive in the caves.
"I am so confident, 1 million percent, because my son is strong," he said. "He is an athlete. He is a footballer. "
CNN's Kocha Olarn reported on Chiang Rai in Thailand, while Euan McKirdy from Hong Kong wrote and reported.