Musk responded to a tweet from a politician who asked how much drilling through the mountain would cost.
The company reportedly said they would offer the first public test ride with the electric transport system inside the tunnel.
He's however building a similar tunnel across town designed to whisk baseball fans to and from Dodger Stadium.
"Boring Company just broke through the other side" tweeted Musk; posted a video of a digging machine.
The first tunnel, under Hawthorne in the Los Angeles area, is "almost done" said Elon Musk.
Tesla shares sank last week after the US SEC accused Musk of securities fraud.
Musk and Tesla to pay $20 million each, bring in two independent directors to settle US SEC charges.
Tesla, Musk will each pay $20 million to financial regulators as he steps down as chairman.
SEC said Musk issued "false and misleading" statements and failed to properly notify regulators.
British diver who helped with Thai cave rescue sues Tesla CEO Elon Musk for accusing him of being a paedophile
Musk called Unsworth a "pedo" in a tweet to his 22.5 million followers after Unsworth criticized Musk in a 13 July television interview with CNN about the rescue. Musk and engineers from his rocket company, SpaceX, built a small submarine and shipped it to Thailand to help with the rescue. The device wasn't used and in the interview, Unsworth called it a "PR stunt" and said it wouldn't have worked to free the boys who were trapped in the flooded cave.
Experts say that Musk’s handling of the private bid may force the board to assert its independence.
Musk called questions posed by two analysts 'boring' and 'boneheaded', and refused to answer them.
Shares of Tesla fell over 3.5 percent after Musk directed abuse on Twitter at a British caver.
On 23 June, 12 young soccer players, along with their coach, got stuck inside a cave in Thailand.
Musk earlier announced that engineers from his SpaceX and Boring Company would head to Thailand.
Elon Musk had mentioned about building a 'cyber dragon' in a tweet a while back.
Elon Musk’s ‘Not A Flamethrower’ flamethrower has now arrived in the hands of consumers, and it's divided the internet in two
The new product by Boring Company is raising a lot of security concerns, while Musk is advertising it as ‘best for BBQ’.
This other-worldly Twitter conversation is what we are here for today.
Elon Musk is angry at the media and has gained support on a Twitter poll for plans to create a website evaluating journalists’ credibility
Criticising the coverage of crashes of Tesla cars and pricing, Elon Musk gained the support of more than half a million people on Twitter to evaluate media
Elon Musk tries to win hearts in Los Angeles to attempt at digging a tunnel beneath a part of the city
The Boring expansion comes as Elon Musk wrestles with production problems for the rollout of the Model 3 sedan at Tesla, his electric car business.