Elon Musk said on Thursday that an underground tunnel developed by his privately held The Boring Company will open on Dec. 18 — an eight-day delay from the initial date.

The billionaire entrepreneur also claimed the postponed date will be "more than a tunnel opening," pointing to "modded but fully road legal autonomous transport cars."

Elon Musk on Twitter: Boring Company product launch on Dec 18. More than a tunnel opening. Will include modded but fully road legal autonomous transport cars & ground to tunnel car elevators.

The tunnel, in the southern suburb of Hawthorne in the Los Angeles area, was initially slated to open on Dec. 10 with free rides for the public scheduled the next day. The high-speed transit tunnel can whisk vehicles from place to place at up to 150 miles per hour.

Elon Musk on Twitter: Opening event that night & free rides for the public next day

Boring, in a separate Twitter post, said the company needs "another few days to finish the snail habitat" and will provide details soon.

The Boring Company on Twitter: Tunnel opening now set for 12/18. Need another few days to finish the snail habitat. More details soon!

— Reuters contributed to this report.