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

The deal has not been finalized and the terms could change, says the WSJ. At $20 billion, the valuation would be a significant increase over 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 transport 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 networks of tunnels under Nashville and Dubai. The WSJ says the startup has also pitched projects in Baltimore, Chicago and Los Angeles.

The startup spun off from SpaceX in 2018. SpaceX recently had the largest initial public offering in its history, but its stock price has fallen significantly since then.