Musk didn't say where the breakthrough happened, but he recently announced he'll be giving a public tour of his latest tunnel project, which is under construction beneath the city of Hawthorne.
Musk envisions building an entire system of underground tunnels beneath Southern California, whisking traffic on electric skates at speeds up to 150 mph.
Musk's public tour of his Hawthorne tunnel is scheduled to happen on Dec. 10.