SEATTLE (AP) — Seattle's giant tunnel drilling machine is nearing the halfway mark on its underground route from Sodo to South Lake Union.

A Department of Transportation spokeswoman told the Seattle Times the drill known as Bertha was close to Pike Place Market on Friday. The machine was less than 100 feet away from reaching the midpoint on the Highway 99 tunnel project.

Studies show Bertha will have to work its way through a mix of clays, cobbles, silt and gravel in its current downtown Seattle location. It could also face boulders as wide as 8 feet.

The four-lane tunnel, which is replacing the Alaskan Way Viaduct, is expected to be complete in 2018.

Contractor Seattle Tunnel Partners has taken several maintenance stops this year to protect the steel machine from abrasion.