Rescue teams were pinning their hopes on a new route north of the entrance of Tham Luang cave in Chiang Rai's Mae Sai district believed to be a safe haven for 12 boys and their football coach who went missing on Saturday.
The teams are still hopeful they will find the team members alive.
Deputy national police chief Wirachai Songmetta led 132 police and rescue workers to survey Doi Pha Mee, which is north of the main entrance of the cave to search for possible points of access to the cave.
The change in the search route followed the advice of three British cave divers, who arrived on Wednesday evening to help in the operation.
Currently, the main rescue and search operations are under way to the west of the cave's entrance, with the location inside the cave dubbed "Monk's Series" as the destination.
It was believed the missing people might have opted to venture north of the cave's entrance rather than head to the location in the West dubbed as "Pattaya Beach".
To the north, they will reach an area of dry ground in a chamber which is about one kilometre away from the cave's entrance, compared to Pattaya Beach, which is about seven kilometres away.
Pol Gen Wirachai said residents told him there is a chamber lying in the north of the cave. It is as high as 60 metres and 20 metres wide. This location has become a new focal point of attention because a cave chamber may have a ceiling crevice.
The 132 policemen and rescue workers were divided into four search teams to survey the area separately. One team used GPS to explore locations on the mountain terrain. Another team was led by locals who said they had seen the ceiling crevice. Another team climbed up and surveyed a mossy rock cliff which means there was dampness coming from in the lee of the hill.
Pol Gen Wirachai added three new ceiling crevices had been found during the survey. Two of them were clogged. A drone will be used to explore the third one.
On social media, some netizens also believed that the 12 boys and their coach might be heading north of the cave's entrance to escape rising waters which blocked the entrance.
That would lead them to Doi Pha Mee which provides an area of dry ground for them. The location is near a source of a stream so water levels are not high.
"However, if the boys and their coach were heading north to find a way out, they would face an increasingly steep slope and they will become even more exhausted,'' a Facebook user said, adding that if they continued on in this direction, they would reach a small cave called "Monk's Series''.
"By now, the boys must be very tired and water may be the only form of sustenance. They may be experiencing wet and cold conditions inside the cave.''
Suttisak Soralump, a geotechnical engineer from Kasetsart University, said percussion drilling equipment will be used to bore holes of 10-12 centimetres wide through the hillside of Tham Luang cave so glowsticks and recording devices will be put in to explore the cave inside. It will take one day to drill a stretch of 100 metres to reach inside.
PTT Exploration and Production Plc (PTTEP) was using two drones to survey the exterior structure of the cave and the information will be used to determine where to drill. The drones with 30x optical zooms, fitted with heat sensor equipment can also take three-dimensional pictures.
Thanes Weerasiri, president of the Engineering Institute of Thailand, said plans are to survey ceiling crevices near Doi Pha Mee. People who previously explored the cave inside said there was a steep slope leading up to the ceiling. Drones were being deployed to explore to see how wide the crevices were and whether they could be enlarged, Mr Thanes said.
He also said efforts will be made to drill diagonally into the left side of Tham Luang cave near so-called Pattaya Beach, a dry ground chamber where the missing people might have been waiting for help.
But before the drilling gets under way, electromagnetic waves will be used to examine the rock structures, he said.
Persistent rain overnight raised water levels inside Tham Luang cave and rescuers were forced to pause their search for 12 boys and their football coach. The five-hour storm from Wednesday into the early hours Thursday pushed water levels in the cave higher.
Floodwaters inside Tham Luang cave, exacerbated by continued rain, are blocking attempts to reach the teen footballers and their coach.
Authorities said the 10-kilometre-long cave in Tham Luang cave had four chambers and floodwater had almost reached the main entrance of the cave early Thursday. Consequently, rescuers had to remove their equipment from the cave and wait for the flood level to subside. Local weathermen expected the rain would stop late in the morning.
Outside the cave, 132 police officers were deployed with sniffer dogs trying to find other openings into the cave. Thirty-two US specialists had arrived at the cave from Hawaii and were considering boring into the cave to hopefully save the missing people.
"Divers are ready for another operation once the water level goes down," a Royal Thai Navy Seal posted in a Facebook message.
The boys, aged 11-16, are members of a local football team nicknamed the "Wild Boars". They and their 26-year-old coach entered the cave in Mae Sai district, Chiang Rai after football practice on Saturday. The mother of one of the players raised the alarm when her son did not return home. Bicycles, shoes and backpacks belonging to the footballers were found near the cave entrance.
Prime Minister Prayut Chan-o-cha will visit the rescue site in a show of moral support for the 13 people trapped in the flooded cave, their families and rescuers.
- Earlier (with video): Drilling started to drain more water
- EDITORIAL: Clear high-ranking hangers-on from the cave