Elon Musk’s tunneling startup The Boring Company is in talks to raise a $4 billion funding round at a valuation of $20 billion, according to The Wall Street Journal.

The deal hasn’t closed and the terms could change, the WSJ says. At $20 billion, the valuation would be a significant increase from The Boring Company’s $5.7 billion valuation in 2022.

The startup has already built a network of tunnels under Las Vegas, through which Teslas shuttle customers to different stations. But tunnel workers have suffered serious injuries, and Nevada regulators said last year that The Boring Company violated environmental regulations nearly 800 times.

The Boring Company has announced plans to build tunnel networks under Nashville and Dubai. The WSJ says the startup has also pitched projects in Baltimore, Chicago, and Los Angeles.

The startup spun out of SpaceX in 2018. SpaceX recently had the largest IPO ever, but its stock price has taken a significant dip since then.