Elon Musk’s Boring Company is reportedly discussing a $4 billion fundraising round that would value the tunnelling startup at $20 billion. If completed, the deal would substantially increase its valuation and underline investor interest in its plans to expand underground transport networks, despite regulatory and safety concerns.

The proposed valuation is far above the company’s $5.7 billion valuation in 2022, although the deal has not closed and its terms may change. The company operates Tesla shuttle tunnels beneath Las Vegas and has announced projects in Nashville and Dubai, while also pitching schemes in Baltimore, Chicago and Los Angeles; Nevada regulators said it breached environmental rules nearly 800 times last year, and workers have suffered serious injuries.