Musk said the aluminum shell of the so-called submarine had high thermal conductivity, meaning it is able to allow heat to pass through it with ease, and therefore provide insulation to the person inside.

The vessel’s structure was made from the liquid oxygen transfer tube of a Falcon rocket, manufactured by Musk’s space exploration business SpaceX.

The Tesla chief executive on Sunday shared a video via Twitter, demonstrating a “tiny, kid-size submarine” being used to transport a person across a pool.

The submarine weighs 40 kilograms when dry, and Musk even raised the possibility of creating a compartment to add a music player to soothe the anxiety of the boys traveling in it.

Rescuers have ramped up efforts to save the boys and their soccer coach, who were discovered trapped inside the flooded Tham Luang cave in the Chiang Rai province last week.

Officials had feared huge delays to the operation due to expected heavy rainfall that would have contributed to further flooding. A diver who had served in the Thai navy died last week from a lack of oxygen after delivering air tanks to aid the rescue mission.

So far, four boys out of the 12 trapped are confirmed to have been rescued from the cave. The rescue efforts resumed Monday following a 10-hour pause that was called last night to replenish scuba tanks with oxygen. A fifth boy has been rescued too, according to multiple reports Monday.

Musk said it would take 17 hours to deliver the mini-submarine. “Hopefully, (it will be) useful,” he said on Twitter. “If not, perhaps it will be in a future situation.”

On Saturday, he sent engineers from two of his companies, SpaceX and tunneling firm The Boring Company, to examine the situation and see if there was anything they could do to help.

Musk raised the possibility of pumping water through the cave to enable a safe exit and of inserting a nylon tube into the cave to inflate it with air “like a bouncy castle.”

The proposed submarine is the latest in a long line of the entrepreneur’s wacky ideas and promises. He has put his name behind everything from and to building the .