If the rain god helps us, then we may be able to work fast. Officials have said storms forecast for Chiang Rai province in Thailand's far north had factored into their decision to go ahead with a complicated and unsafe plan to have the boys and their coach dive out of the cave.
Weather forecasters warned heavy rain could hit the area through the week.
The boys, who have not been identified, were kept away from visitors due to concerns about possible infection.
It is reported that the remaining four youngsters and their coach will remain in the cave for another night. Musk said his submarine has eight handles, four in the front and four in the rear, and four air tank connections.
The success of the initial evacuation raised hopes that all will be out soon, although officials said could it take up to four days to complete.
Rescue teams had installed a static rope along the dive path, giving the boys a guide, and experienced divers were reportedly positioned at various points along the way for assistance.
Multiple sources with knowledge of the operation told Reuters the operation to rescue more of the nine boys and their coach was underway, although no official confirmation was issued. Thai Navy SEALs and global cave diving experts brought out four boys Sunday, and the operation is continuing today.
Each boy is being accompanied by two divers through the dark and complex cave system. None of those kids will have to spend any time inside the tiny submarine, but it could be developed further for similar rescue missions in the future.
The rescue mission has been a huge operation, led by the Royal Thai Navy's SEAL unit, and supported by a cast of hundreds.
Thailand's Interior Minister Anupong Paojinda said the divers who took part in Sunday's rescue were also conducting the current operation.
Chiang Rai acting Gov. Narongsak Osatanakorn said they "hope to hear good news in the next few hours".
He said the health of the five people remaining in the cave was "still good" and rescuers would resume an operation to retrieve them in about 20 hours' time.
But four boys had 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.
The potential for rising water and the dwindling oxygen levels added to the urgency of getting the team out.
Teams have used high-powered pumps to empty more than 100 million of gallons of water out of the cave.
Narongsak said Saturday that experts told him new rain could shrink the unflooded space where the boys are sheltering to just 10 square metres (108 square feet).
The boys and their coach went exploring in the massive Tham Luang Nang Non cave on June 23 after a soccer practice, and were cut off when a rainstorm flooded the cave. More than 1,000 journalists from across the world have descended on northern Thailand to report the story.
He also posted photos and videos of a team of divers testing the pod in Palisades Charter High School's swimming pool in LA, including one simulating the pod being manoeuvred through a narrow passage and another showing a man coming out of it safe and dry. If the tests were successful, the sub was to be placed on a 17-hour flight to Thailand.
"We have more operating personnel, and we have more expertise than yesterday", Narongsak said.