The Boring Company, Elon Musk’s tunneling startup, is in discussions to raise around $4 billion in new funding that would value the firm at approximately $20 billion, according to sources familiar with the talks. The round has not yet closed and terms could still change.

Steve Davis, president of the Boring Company, did not respond to requests for comment. The company asserts its tunnel boring machines can dig underground more cheaply than traditional construction firms. It spun off from SpaceX in 2018.

The Boring Company operates a transportation network beneath the Las Vegas Strip, where Tesla vehicles ferry passengers between the Las Vegas Convention Center and other points as part of its Las Vegas Loop system.