However, the first four boys were safely brought out of the cave on Sunday, tethered to rescue divers.
The four boys rescued on Sunday are now eating normal food and walking around, while the four extracted on Monday are being given soft food.
Chiang Rai Gov. Narongsak Osatanakorn says the rescue mission began at 10.08am and involves 19 divers.
A health official earlier described the mental and physical condition of the eight rescued so far as "good", adding they will be kept in hospital for around a week.
"They are all in good spirits and happy to be out", Jedsada Chokdamrongsuk, permanent secretary of the public health ministry, told reporters. The boys then walked from Chamber 3 to the mouth of the cave, which has been mostly drained over the last few days despite heavy rains.
The perilous mission saw the boys having to use scuba masks as they were led through the narrow tunnels to safety.
Nevertheless, the boys would remain in quarantine in hospital until doctors were sure they had not contracted any infections from inside the cave.
None of the first group of four boys to emerge has a fever, and the two boys that had lung infections are improving.
"The kids are footballers, are strong and have high immunity", Jesada said when asked why they survived so long.
Our condolences to Doctor Richard Harris, one of the leading rescuers whose father just passed away hours after his rescue mission had been completed.
Acknowledging the 38-year-old former military diver who died while preparing the rescue, Prayuth said: "his honour, sacrifice and legacy will forever be in our hearts".
According to an account given to the Guardian by three Australian divers involved in the mission, rescue workers were reportedly still inside when the pump failed, meaning water levels started to rise. They began again at 11 am on Monday after the teams had stopped for the night to rest and to replenish equipment and oxygen supplies.
LILLIAN SUWANRUMPHA/AFP/Getty Images Thanaporn Promthep, mother of one of the 12 missing boys, displays an image believed to have been taken in 2017 of her son Duangpetch Promthep, nicknamed "Dom" (with the light) and his football coach Ekkapol Chantawong, right, after hearing the news the group was found on July 2, 2018.
Officials involved in the rescue of the soccer team from the Tham Luang cave complex have politely declined to deploy the submarine brought over by Tesla CEO Elon Musk to assist with the extraction of the kids.