And so it begins: Construction on a highway tunnel beneath downtown Seattle was brought to an abrupt halt Friday after workers encountered a mysterious object of unknown origin lurking underground.

An $80 million, five-story-tall boulder-busting machine named Bertha that was brought in to drill the two-mile Highway 99 tunnel was unable to break through the object and was shut down on Saturday after reaching the 1,000-foot marker.

The Seattle Tunnel Partners contracting team and Transportation Department engineers are working with experts to identify the unexplained obstruction, and are exploring the possibility of drilling down 60 feet into the object from the street above.

The area being tunneled contains a mixture of native soil and fill that was put in place over a century ago.

Speculation as to the object's true identity include an Ice Age-era boulder or a pioneer-era rail car.

"We don't know whether it is man-made or natural," a WSDOT official told the Seattle Times.

Or neither.

[video via KING5]