Some SpaceX investors are concerned about CEO Elon Musk's other ventures, especially when their capital is deployed for his latest pet project, The Boring Company, The Wall Street Journal reported Monday.

The Boring Company's first tunnel, which is set to open Tuesday after a delay, is being constructed at SpaceX headquarters in Southern California and has been funded by money intended for the rocket venture -- not tunnels -- sources told The Journal.

SpaceX does own a 6 percent stake in The Boring Company, a spokesperson said, "calculated based on the value of land, time, and other resources contributed since creation of the company." That's despite the company's board never voting to deploy the cash in the first place, according to the Wall Street Journal.

The equity was always in place, though investors were unaware, the spokesperson said.

A Boring Company spokesperson declined to comment.

--This post originally appeared on Business Insider.