THAM LUANG CAVE, Thailand >> As the world watched rescuers struggle to save 12 boys and their soccer coach trapped in a cave in northern Thailand, tech billionaire Elon Musk dreamed up a “kid-size” submarine to help. It turns out they didn’t need it.

Today, the rescue operation had succeeded in saving all 13 people trapped in the Tham Luang Cave. Earlier this week, eight of the soccer teammates were evacuated from the flooded cave, a mission that required guiding the boys through passageways filled with water.

It was a daunting task. On Friday, a former Thai Navy SEAL lost consciousness and died while placing air canisters on a route within the cave being used in the rescue effort.

Once the last four boys, their soccer coach and military personnel were evacuated today, Thai officials said in a Facebook post that the mission was complete. It had taken 18 days, dozens of divers and hundreds of volunteers, but Musk’s mini-submarine invention was left on the sidelines, a Thai official said.

Musk himself was on the scene. He wrote in a tweet early today local time that he had just returned from a large cavern in the Tham Luang cave complex where he had deposited the quickly assembled sub.

But on Tuesday, the head of the search operation, Narongsak Osottanakorn, until recently the provincial governor, rejected the notion that Musk’s custom-made submersible was suitable for the extraction.

“I assure you that the equipment he brought to help us is not practical for our mission,” Narongsak said. “Even though the equipment has state-of-the-art technology, it does not fit our mission in the cave.”

Over the weekend, Musk dispatched 10 engineers from his companies to assist in the rescue effort in Thailand. They sent engineers from Tesla, Musk’s electric vehicle company; SpaceX, his rocket company; and the Boring Co., which specializes in tunneling and construction, a Boring Co. spokesman said Saturday.

Musk had mused on Twitter that they could build a “tiny, kid-sized submarine” with a part from a Falcon rocket, which SpaceX produces. He had said that a transfer tube, which helps move liquid oxygen to fuel the rocket’s engine, could serve as the body of the submarine.

Musk’s representatives did not make him available for comment, but a SpaceX spokesman said that divers with the mission had signed off on the efforts.

“Multiple divers involved with the rescue said the mini-sub design was viable, and their feedback directly informed our engineering and testing,” he said.

In his tweets amid the rescue, Musk presented his own evidence that team leaders had welcomed his help. In an email he shared from Richard Stanton, one of the first two British cave divers to reach the soccer team, Stanton had asked him to “please keep working on the capsule details.” A spokesman for Stanton said today that the cave proved to be too narrow for the mini-submarine.

Soon after the cave rescue was complete, it became clear that Musk would be sending many more employees to Asia in the coming year. Tesla today announced an agreement with Chinese authorities to build a battery and automobile factory in Shanghai, its first plant outside the United States.