The Highway 99 tunnel drill started digging again under Belltown on Monday night, after 300 cutting teeth were replaced in a maintenance stop.

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Tunnel-boring machine Bertha has restarted with hundreds of new cutting teeth and only a half-mile left to dig before reaching daylight.

Excavation resumed Monday night, after divers operating in high-pressure conditions replaced 300 of 500 teeth on the drill’s rotary disc, during a planned 12-day stoppage. These teeth weigh 75 pounds each, and they erode in abrasive Seattle soils.

Seattle Tunnel Partners must replace the teeth periodically, or risk being unable to scrape dirt and push forward.

The top of the machine is now 160 feet below Third Avenue at Blanchard Street, said a brief update by the Washington State Department of Transportation. Barring soil or mechanical problems, it should reach the finish line near Seattle Center by midyear, followed by completion of the road decks and lights, signs and other electronics.

The four-lane, $2.1 billion Highway 99 tunnel is expected to open in early 2019 for traffic, three years late. Legal fights over who pays for a potential half-billion dollars in cost overruns may stretch years longer.