Billionaire tech entrepreneur Elon Musk built a special kid-size submarine that simply wasn’t practical for the Thailand cave rescue, authorities said Tuesday.

The SpaceX and Tesla CEO showed off the small metallic pod on Twitter over the weekend as it was tested by his engineers in a Los Angeles high school swimming pool — and then flew with it to Thailand, hoping it could help rescuers working tirelessly to free the 12 soccer players and their coach stuck in the partially flooded Tham Luang cave.

But after visiting the rescue site, Musk was “politely” told the mini-submarine wouldn’t be of any use.

“Although his technology is good and sophisticated, it’s not practical for this mission,” former Chiang Rai governor and rescue mission chief Narongsak Osatanakorn said Tuesday, according to The Guardian.

The Thai navy SEALs subsequently announced that the remaining four boys and their coach had been brought out safely.

Musk left the sub, which is made of rocket parts and named Wild Boar after the kids’ soccer team, at the cave site — just in case it can be useful in the future.

Thailand’s prime minister, Prayut Chan-o-cha, said he was “highly appreciative” of Musk’s initiative and “personally touched” that Musk traveled to Thailand with his “ingenious solutions.”