The Boring Company announced Wednesday that it had raised $675 million in a Series C funding round, valuing the firm at $5.675 billion.

The funding round was led by venture capital companies Vy Capital and Sequoia Capital, with several other VCs and real estate firms also taking part.

Founded by Musk—the world’s wealthiest man—the Boring Company is working to create ultra-fast underground transportation hyperloops that it says will “solve traffic, enable rapid point-to-point transportation, and transform cities.”

To date, the company has completed two test tunnels in Hawthorne, Calif., and constructed a so-called loop system that connects the Las Vegas Convention Center buildings. 

Billionaire Musk, who also heads up space exploration company SpaceX and neurotechnology firm Neuralink, has been making waves in recent weeks with suggestions that he intends to buy Twitter.

Earlier this month, it emerged that Musk had bought a 9% stake in Twitter, prompting shares of the company to surge by more than 20%.

In a cryptic tweet on Thursday, Musk fueled speculation that he has plans to make a tender offer for the social networking firm in the event that the board rejects his “best and final” offer to buy 100% of the company for $43 billion.

Twitter has taken action to resist Musk’s hostile takeover bid, with management enacting a limited-duration shareholder rights plan—also known as a “poison pill”—which would help it to repel Musk’s offer to buy the company and take it private.

Meanwhile, Musk’s electric vehicle company Tesla reported better-than-expected first-quarter earnings on Thursday, publishing a Q1 report that outlined “another record quarter for Tesla by several measures such as revenues, vehicle deliveries, operating profit, and an operating margin of over 19%.”