Crews will have to cross the tectonic boundary that separates the North American and Pacific plates, boring through a jumble of fractured rock formations and a maze of earthquake faults, some of which are not mapped.

It will be the most ambitious tunneling project in the nation's history.

State officials say the tunnels will be finished by 2022 — along with 300 miles of track, dozens of bridges or viaducts, high-voltage electrical systems, a maintenance plant and as many as six stations. Doing so will meet a commitment to begin carrying passengers between Burbank and Merced in the first phase of the $68-billion high-speed rail link between Los Angeles and San Francisco.