ELON Musk has developed a mini-submarine and left it in the cave where four Thai boys and their coach remain trapped, saying it may be useful in the future.

The billionaire Tesla CEO posted to Twitter this morning saying he had just returned from inside Cave Three, and brought the rescue device with him - which he named "Wild Boar", after the kids' soccer team.

Cave Three is roughly 2km from the entrance of the cave network and is the base for Thai rescuers.

There is no indication that Thai rescuers are going to use Musk's device.

The tech entrepreneur designed an escape pod for the complex rescue effort, tweeting over the weekend that he was working with a team from his Space X rocket company to build a "tiny kid-size submarine" to transport the boys out of the cave.

He posted a video yesterday showing a simulation of the device moving through a narrow passage.

The person inside need not swim or know how to use oxygen bottles.

 

The cave tunnels the remaining five must navigate through get as narrow as 38 centimetres tall - barely longer than an average ruler.

Musk said based on "feedback from Thailand," the engineers built the metallic pod using a large silver tube meant to be affixed to a SpaceX Falcon 9 rocket.

Earlier in the week, he described the device as "basically a tiny, kid-size submarine using the liquid oxygen transfer tube of Falcon rocket as hull".

He said it was "light enough to be carried by 2 divers, small enough to get through narrow gaps" and "extremely robust".

The device is outfitted with oxygen ports and a nose cone to protect it from impact with rocks.

Musk's pod was one of various extraction methods proposed by experts, but the original plan to swim the boys out was decided as the best option due to the unpredictable weather conditions.

Rescuers had attempted to dig more than 100 holes, with 18 of them looking promising. But Mr Narongsak said he was not sure they could reach the boys, with the deepest reaching only 400 metres below the surface and the group trapped more than 600 metres down.

The British Cave Rescue Council said drilling would be too difficult because the equipment could not get through the dense foliage, there was no exact known target for the drill and the procedure could destabilise rocks above the children or block passages and increase flooding.

Four more boys were rescued from the Thai cave in a rapid-fire operation last night.

The head of the operation would not confirm whether all five could come out today, raising questions over whether coach Ekkapol Chantawong could be left behind to spend a lonely night in the cave.



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