The Transportation Department said Wednesday the 326-foot-long machine it calls Bertha has drilled 359 feet.
Spokeswoman KaDeena Yerkan says the trailing gear has disappeared into the tunnel.
Yerkan says Bertha is operating well but is still in the initial phase with about 9,000 feet to go.
The world's largest tunnel boring machine is creating a tunnel nearly 58 feet in diameter as part of the $3.1 billion project to replace the Alaskan Way Viaduct, the double deck highway along the downtown Seattle waterfront.
Digging began July 30 but soon shut down for about a month in a labor dispute before it resumed on Sept. 23.