With eight boys already rescued, Thailand on Tuesday sought to bring out the last five people who have been trapped inside a flooded cave since June 23.
Monday's operation, which rescued four boys, went faster and more smoothly than the previous day's, which also rescued four boys, raising hopes that the end of the harrowing 17-day ordeal could finally be within sight.
The group of a dozen young football players, aged 11 to 16, and their 25-year-old coach were trapped inside the cave when flash floods blocked their way to the exit.
Their predicament has gripped Thailand and much of the world.
Details of the rescue mission have been limited, but it is said to involve scuba diving through narrow passageways that remained totally submerged in water.
Early Tuesday, entrepreneur Elon Musk tweeted, apparently from Thailand: "Just returned from Cave 3. Mini-sub is ready if needed. It is made of rocket parts & named Wild Boar after kids' soccer team. Leaving here in case it may be useful in the future."
Musk had offered assistance earlier in the rescue mission, sending engineers from two of his companies to provide technical support.
But it was unclear whether Musk's apparent arrival in Thailand - and his "tiny, kid-size submarine," which he has previously displayed on social media - signalled a greater involvement than had been earlier disclosed.
All of the boys who have been rescued are in hospital in Chiang Rai and are "safe and healthy," provincial governor Narongsak Osottanakorn, who also heads the rescue effort, told reporters late Monday.