Elon Musk is serious about using tunnels to beat traffic.

Musk raised eyebrows in mid-December when he announced plans via Twitter to buy a tunnel boring machine and "just start digging." He promised to call the effort The Boring Company.

"Traffic is driving me nuts," he tweeted.

Since then it's been unclear whether Musk was momentarily frustrated with Los Angeles traffic, or would actually follow through on the project.

Musk is already busy at the helm of two companies, SpaceX and Tesla (TSLA). He's also the co-chair of OpenAI, a business adviser to President Trump and the father of five children.

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But early Wednesday morning on Twitter, Musk called his tunnel progress exciting, and said he planned to start digging in a month or so.

Musk has had a long standing interest in tunnels. A year ago at a Hyperloop event at Texas A&M, Musk described tunnels as a way to alleviate congestion in cities.

"This is really a very simple and obvious idea," Musk said. "You could have tunnels at all different levels. You could probably have 30 levels of tunnels and completely fix the congestion problem."

Traditionally, tunneling has been very expensive. It can cost hundreds of millions of dollars per mile. However, Musk has shown that he can dramatically reduce the costs needed to pull off complex engineering challenges. His rocket company SpaceX has earned praise for its low prices.

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Musk's interest in tunnels also sets up another potential battleground with rival Jeff Bezos. The two have dueling rocket companies, and have at times sparred on Twitter over their accomplishments in space.

Earlier this month, Amazon (AMZN) received a patent for a system of delivering packages via tunnels. While it's unclear if Amazon will actually follow on the plans, Musk isn't the only billionaire with tunnels on his mind.