If the tests are successful, the sub would be placed on a 17-hour flight to Thailand.
As of Sunday, four boys were confirmed rescued from the cave, before the mission was paused, according to the BBC.
Authorities temporarily stopped rescue efforts on Monday to replenish air tanks along the cave's perilous exit course.
The total number of boys pulled from the cave Monday is four, following the four that were pulled Sunday. "They have not been moved to Chiang Rai hospital yet", Tossathep told Reuters. The group is now the focus of the worldwide rescue teams.
The dramatic and risky rescue at the Tham Luang cave in the northern province of Chiang Rai was launched on Sunday and four boys were brought out that day. Musk, head of Tesla and the Boring Company, tweeted that the tube would be "light enough to be carried by 2 divers, small enough to get through narrow gaps" and "extremely robust".
The four boys emerged by 8 p.m., quite a bit ahead of schedule -and all the more remarkable because the boys are novice swimmers, with no diving experience. Their condition was not immediately clear.
Earlier another official had said six children had been taken out of the cave.
Authorities in northern Chiang Rai province began the risky mission to bring out the 12 boys and their coach earlier on Sunday.
"The operation went much better than expected", said Chiang Rai acting Gov. Narongsak Osatanakorn, who is overseeing the mission.
The operation began at 10 a.m.
The operation began just hours after the group was reported missing but the heavy rains slowed down the search.
The boys and their coach, whose team is known as the Wild Boars, became stranded when they were exploring the cave after a practice game on June 23. They eventually found a dry landing spot, where they waited for nine days before being found by two British divers.
"The boys were wearing full-face masks and were able to breathe normally during the operation", he added.
More than 90 divers were involved in the rescue operation, which will continue today.
The rescue team included both divers who participated in Sunday's operation and new divers to replace those exhausted from the previous day's mission, he said. "We have set the plan for four people so if they want to take five out (at the same time), then they need to change the plan". Teams had to bring in industrial pumps for the operation since parts of the cave were flooded by the rains.
The mission was only launched on Sunday morning because floodwaters inside the cave were at their lowest level in days.
Rescue chief Narongsak Osottanakorn said the four who escaped were "safe" but released few details about their condition or identities.
The ambulance drove toward a helipad, where a helicopter was seen taking off shortly after to the cheers of the crowd below.
He says: "If we keep on waiting and the rains come in the next three or four days, our readiness will decrease".
Initial euphoria over finding the boys alive quickly turned into deep anxiety as rescuers struggled to find a way to get them out.
Thirteen medical teams were stationed outside the cave - each with its own helicopter and ambulance - one for each of the 12 boys and their coach.
"Today is D-Day", Narongsak had earlier told reporters.
Engineers have been diverting the water flow into the cave over the past week and pumping it out at a rate of 180,000 litres an hour in an attempt to make it easier for the boys to escape and reduce the amount of time they must spend underwater.