So, never one to keep a single project in mind, Musk started down two paths. For the first, he tapped Wing Inflatables.

The SpaceX contractor is based in Arcata, California, 350 miles north of San Francisco. “We manufacture inflatable recovery parts for SpaceX,” says CEO Andrew Branagh. “So we have a relationship. When Elon had an idea, he asked our engineering team to get a hold of us." So they hopped on a conference call. “Elon Musk was on the call,” Branagh says. “He was very visionary. I was impressed. They were open to our ideas."

The result of the collaborative brainstorm was a red kevlar pouch of sorts, designed to carry the boys. The "inflatable tube with airlocks," as Musk called it, could be pulled along, stretcher style, by a trained diver, freeing the boys from needing to learn how to dive or use scuba gear.

"They are like a bell chamber," Branagh says. Just as a church bell tower contains or releases sound by closing or opening a series of vents, the inflatable tube with airlocks uses air pressure to control the buoyancy, necessary for moving through a cave complex where you occasionally have to drop down to keep moving forward. When you turn parts of the pods one way, it allows them to float. "When you turn them another way, they sink." (To keep the boys calm during a long, claustrophobic voyage, Branagh suggested valium.)

They moved fast and worked hard. "We started with a concept at 8 am and had a prototype in the pool being tested the same day,” Branagh says. “Then the team stayed until 1 in the morning and built more units.” Enough units to get all 12 boys, plus their coach, out of the cave and into the daylight. The following afternoon, Musk’s jet touched down in Arcata to pick up the potential life savers. “A third of the factory went to the plane to wish it luck,” Branagh says.

The pouches that went into the jet, however, never appeared in the public light, and it’s not clear where they ended up. Musk never mentioned them on Twitter. While Branagh and his team were racing to put them together, Musk was hedging his bets with another design approach.

Made from an oxygen tube designed for a SpaceX Falcon rocket, the bullet-shaped capsule followed the same principle as the pouch the Wing team devised, an air pressure–controlled vehicle of sorts for the boys to lay inside while the pro divers guided them to safety. Musk called it a "kid-sized submarine” and said it was 12.2 inches in diameter, skinny enough to fit through the narrowest “choke hold” of the passage. He called it Wild Boar, in honor of the boys’ soccer team.

"Good for rescuing vulnerable patients in dangerous environments," he tweeted, "particularly if water, toxic gas or dangerous bacteria/viruses present, as patient would remain dry & at [standard] air pressure entire time." And because Musk is always thinking on a few tracks at once, he noted, “with some mods this could also work as an escape pod in space."

As the engineering process sped along, Musk updated the public with a steady stream of tweets. The aluminum capsules could use a thin layer of neoprene insulation, he said, and their buoyancy would be controlled by strapping diving weight belts around them.

Musk also considered one follower’s suggestion to add a music player to the setup. "Yeah, that sounds cool,” he replied. “Music makes things better. Calms the mind. Adding padded wall pockets for a hand radio & phone/music player."