He expected it to arrive around 11 p.m. Monday, local time.
"Experts' assessments indicate that today is the most ready we can be", Choorat Panngao, Provincial Police Region 5 deputy commander said.
The same divers who rescued the children on Sunday went back in to collect the next four on Monday, with the country's interior minister Anupong Paojinda saying they would conduct the next operation as they already knew of the cave's conditions and how to safely bring the children out.
An official briefed on the rescue operation said Monday the rescued kids haven't yet been served the chicken dish but are starting instead with lighter blander food such as diluted porridge.
Dive tanks are needed on the journey in and out - the average tank holds about 2200 litres of air, and a skilled diver uses between 15-20 litres per minute near the surface, with that amount increasingly rapidly at depth.
Eight other boys, aged between 11 and 16, and their 25-year-old coach remained stranded as of Monday morning.
The next day (7th July), it was decided by the rescue operation chief Narongsak Osottanakorn that it was not yet safe for the boys to dive out of the cave.
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Thai authorities said the parents of the four rescued boys are still being kept from physical contact with them due to fear of infections. The boys who were removed were considered the weakest, reports said.
The "Wild Boars" soccer team and their coach became trapped on June 23 when they set out to explore the vast cave complex after soccer practice, when a rainy season downpour flooded the tunnels.
The eight boys who were rescued through Monday had to travel about two and a half miles with oxygen tanks and were accompanied by two professional divers each. They were stranded by flash floods that caused water levels to rise, making it hard to get out.
The billionaire entrepreneur has been tweeting proposals and offering his services to the Thai government in its efforts to extract the 12 boys and their soccer coach from a partially flooded cave. Earlier in the day, he went to Phuket to check on the search operation of tourists who were missing after a boat capsized off the resort island's coast on Thursday.
The death Friday of a former Thai navy SEAL underscored the risks.
Four of the boys were rescued on Sunday, and authorities are now working to replenish air tanks along the cave's treacherous exit route.
The team had to pause rescue efforts as weather conditions worsened and made the narrow cave system more hard to navigate.
In the clip shared by Musk above, you can see the submarine being tested by a team of divers at a high school pool in Los Angeles.