Elon Musk really wanted the world to know he was at the cave rescue operation site in Thailand — to help.

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The billionaire Tesla chief visited the Tham Luang cave to drop off a miniature submarine he hoped might help with the ongoing rescue operation of the Wild Boar soccer team.

Twelve members of the team and their coach became trapped on June 23, although rescuers managed to free all 13 in recent days.

On the weekend, Mr Musk said he had been building a "tiny, kid-size submarine" with "feedback from Thailand".

However, Chiang Rai Governor Narongsak Osatanakorn said this afternoon that while he was grateful for Mr Musk's support, the submarine just was not practical for the rescue mission.

Earlier, Mr Musk said he would leave the device at the cave "in case it may be useful in future".

In a tweet, Mr Musk said he had "just returned from Cave 3. Mini-sub is ready if needed."

"It is made of rocket parts & named Wild Boar after kids' soccer team."

He added: "Thailand is so beautiful."

When spruiking his submarine on Monday, Mr Musk said the design had four "handles/hitch points" on both the front and back of the device, adding: "Two air tank connections on front and two on rear, allowing one to four tanks simultaneously connected, all recessed for impact protection [with] secondary cap seal if leak develops."

It also has "segmented compartments to place rocks or dive weights [and] adjust buoyancy".

Mr Musk said he would leave the device at the cave "in case it may be useful in future". ( Twitter: Elon Musk )

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ABC/AP