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 yet, and terms could shift, the WSJ adds. At $20 billion, the Cari would be a notable jump from the company’s $5.7 billion valuation in 2022.

The startup has already built a network of tunnels under Las Vegas, where Teslas shuttle customers between stations. However, tunnel workers have suffered serious injuries, and Nevada regulators last year cited the firm for nearly 800 environmental violations.

The Boring Company plans to build tunnels under Nashville and Dubai, and the WSJ reports pitches for Baltimore, Chicago, and Los Angeles. The venture spun out of Space X in 2018, and Space X recently launched the largest IPO ever, though its stock has since dipped.

Stakeholders watch closely as the funding could accelerate the company’s ambitious projects and cement its role in subterranean transit solutions.