Enlarge Image Elon Musk/Twitter

The rescue operation to save a Thai youth soccer team from the flooded Tham Luang Nang Non caves in northern Thailand began in earnest on Sunday, with an experienced team of divers rescuing four of the 13 people trapped. On Monday, rescuers retrieved four more boys, bringing the total freed so far to eight.

Before the rescue mission began, Elon Musk tweeted that he was designing an "escape pod" built from "the liquid oxygen transfer tube" of a SpaceX Falcon rocket that could be used as a submarine. 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."

Now playing: Watch this: Thai cave rescue: Elon Musk sends tiny submarine

On Sunday, Musk tweeted a series of videos showing a team of divers testing the pod in the swimming pool at Palisades Charter High School. The tweets show the submarine being slowly ferried underwater by a team of divers.

Testing underwater in LA pool pic.twitter.com/CDO2mtjP2D — Elon Musk (@elonmusk) July 8, 2018

Simulating maneuvering through a narrow passage pic.twitter.com/2z01Ut3vxJ — Elon Musk (@elonmusk) July 9, 2018

The last video shows the process of removing the submarine from the pool and the team unlocking the door, before the passenger, arms crossed, slides out. The diameter of the tube measures just 12.2 inches.

Musk said in a tweet that he expected the mini sub would arrive in Thailand by sometime Monday evening, local time. "Hopefully useful," he wrote. "If not, perhaps it will be in a future situation." The sub, dubbed Wild Boar after the soccer team's name, is ready for use when and if necessary, Musk said in another tweet​, adding that the vessel will remain in Thailand in case it's needed in the future.

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. Thailand is so beautiful. pic.twitter.com/EHNh8ydaTT — Elon Musk (@elonmusk) July 9, 2018

Four boys and their coach remain trapped in the caves. It remains unclear whether Musk's submarine will be used to aid retrieval of the team, but the SpaceX CEO also suggests the submarine could be modified for use as an "escape pod in space."

The team of 12 boys and their coach entered the cave system on June 23, when torrential rain caused the water level within the cave to rise rapidly, forcing them to venture deeper. They eventually found a dry landing spot, where they waited for nine days before being found by two British divers.

Originally published July 8 at 4:43 p.m. PT.

Update at 5:18 p.m. PT: Additional video added showing simulation through narrow passage.

Update July 9 at 5:49 a.m. PT: Added that four more members of the soccer team have been rescued.

Update July 9 at 4 p.m. PT: Added information about sub being delivered to cave.

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