Elon Musk shows off 'kid-sized submarine' he thinks can help in Thai cave rescue (VIDEO)

Elon Musk has released videos of a test of his "kid-sized submarine." He announced the vessel would be built to rescue Thai children trapped in a flooded cave hours before the operation started without him.

The billionaire SpaceX and Tesla CEO announced he was building the submarine on Saturday, saying it would be ready within eight hours. He said its hull is made using the liquid oxygen transfer tube of the SpaceX Falcon 9 rocket. On Sunday, he tweeted videos showing the submarine being submerged in the Palisades Charter High School swimming pool in Los Angeles. Musk said the submarine is already on its way to Thailand, a journey that will take 17 hours in total.

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The operation to rescue 12 boys and their coach from a flooded cave in Thailand kicked off early on Sunday, a couple hours after Musk had predicted his project would be complete and almost a day earlier than he said he would be able to bring the sub to Thailand. By the time he posted the test drive videos, four boys had already been rescued and the operation was suspended for the night.

READ MORE: Four children saved from flooded cave in Thailand, rescue op halted for night (VIDEO)

Musk's submarine appears to be a bullet-shaped container without any means of propulsion, meaning it would need to be pulled and pushed through the cave's narrow openings by divers. According to the entrepreneur, it's only 31 centimeters (1 foot) wide, and thus it is capable of slipping through the submerged tunnels in the Tham Luang cave system, some of which are reportedly too narrow to pass through even wearing scuba diving tanks.

Some skeptical commenters pointed out that the vessel's use in the cave might be stunted by its inflexible design. Besides, it being "kid-sized" would limit its future potential in rescuing anyone larger than a child. Others lamented the lack of any windows to the outside world, which, in a one-foot-wide tube, would make the rescued children even more claustrophobic than the flooded cave and the dark tunnels themselves.

While by showing material evidence of the sub's existence, Musk may have curbed the critics that assumed his initial announcement was nothing but a PR stunt off the back of the Thai drama, it's still unclear if the submarine will be useful in the actual rescue. Still, Musk believes it will remain viable beyond the present circumstances and could be useful in similar situations in the future, or even, with some modifications, as a spacecraft's escape pod.

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