SEATTLE (AP) — The state Department of Transportation and contractors building a highway tunnel under downtown Seattle are trying to find out what has blocked their tunnel-boring machine.

The machine called Bertha ran into something two weeks and was shut down about 1,000 feet into the 1.7 mile project and 60 feet below South Jackson Street and South Main Street.

The $80 million machine is designed to break up boulders, so what it hit is a mystery. It may be a huge boulder left by the last ice age. Others think it could be a buried rail car from pioneer days when the area was filled in.

Engineers are considering drilling down to the object as one of the ways to break up or remove the obstruction.

The 58-foot diameter tunnel is scheduled to be completed by the end of 2015, creating a four-lane route for Highway 99 traffic between South Lake Union and the area south of downtown near the Seahawks and Mariners stadiums.

After removing the obstruction, crews will probably have to inject concrete grout to fill resulting gaps. The machine requires soil resistance to stay on course.

After a slow start over the summer, the machine was averaging 52 feet a day last week before running into the obstruction, said DOT spokeswoman KaDeena Yerkan.

Two weeks ago the drilling effort "was stopped after unanticipated and increasing resistance was experienced, possibly due to an obstruction," said Highway 99 project administrator Todd Trepanier.

The $1.4 billion tunnel contract is part of the $3.1 billion project to replace the Alaskan Way Viaduct along the downtown Seattle waterfront.