Elon Musk announced Sunday that the first tunnel of a proposed underground transportation network across Los Angeles County would open Dec. 10. “The first tunnel is almost done,” Musk tweeted to his 23.1 million followers shortly after 5 p.m. Musk’s Boring Co. is building the tunnel beneath the city of Hawthorne, part of his grand vision for a transportation network that whisks commuters across the county. Last year, Hawthorne officials approved the Boring Co.’s request to tunnel west from Space X headquarters. At the time, they said the tunnel had extended 500 feet. The company has said its technology...