Mr Musk visited the Tham Luang cave complex in Chiang Rai on Monday, where four children and their football coach remain confined to a small air pocket inside the waterlogged cavern.
He had been exploring solutions to save the boys and his SpaceX company had devised, built and tested the “kid-sized” craft before shipping it to Thailand.
“Just returned from Cave 3. Mini-sub is ready if needed,” Mr Musk said on Twitter, sharing a photograph from inside the complex.
“It is made of rocket parts & named Wild Boar after kids' soccer team. Leaving here in case it may be useful in the future.”
However, head of the rescue mission, Narongsak Osotthanakorn, who has already overseen a successful operation by divers to extract 11 boys from the cave, played down the helpfulness of Mr Musk’s efforts.
He acknowledged the help of the team sent to Chiang Rai by the entrepreneur at a press conference on Tuesday, but said the equipment they had brought was not: “practical with our mission.”
“Even though their equipment is technologically sophisticated, it doesn’t fit with our mission to go in the cave,” he added.
A total of 12 boys and their football coach have been trapped underground since June 23, when they went to explore the caves after a match.
The group were not discovered by search teams for nine days and have spent a further week underground waiting for authorities to devise a rescue plan.
Divers have been rushed into action after fears incoming monsoon rains could shrink the size of the air pocket in which the team is surviving.
Four boys were dragged from the cave on Sunday, while authorities were able to rescue a further four Monday.
The team of divers plans on Tuesday to remove the remaining five people from the cavern and complete a mission which has captured the world's attention.
Even the most experienced divers find the route through the caves challenging, where the smallest choke points are little more than the width of an adult’s shoulders.
On Friday, a former Thai Navy Seal who had been assisting with the rescue effort died trying to make his way through the cave system.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies