Ted S. Warren / AP In this photo made with a fish-eye wide-angle lens, the massive boring machine known as Bertha that is drilling a two-mile tunnel to replace Seattle's viaduct is shown before the project began.

The great Seattle mystery has been solved: The object blocking the world’s largest tunnel boring machine isn’t an old steamship or locomotive or alien spaceship – it’s an 8-inch-diameter steel pipe.

The Washington State Department of Transportation said Friday that the steel pipe that stopped Bertha, as the borer is nicknamed, on Dec. 6 is a 119-foot-long well casing installed in 2002 to study groundwater movement under downtown Seattle.

“We need to investigate further to see if there are other factors that could have contributed to the blockage,” Matt Preedy, a deputy administrator on the project, said in a statement.

When it got stuck, big Bertha was only about 1,000 feet into a two-mile-long boring journey under Seattle for the tunnel that will replace the city’s waterfront viaduct – a necessity after damage from a 2001 earthquake. The borer is 60 feet below the surface in an area that actually was above ground when the city was founded but got buried in waves of industrial growth and redevelopment.

The mystery had intrigued Seattleites, with some speculating that an old locomotive or steamship boiler might have been buried in the fill.

"It could be Jimmy Hoffa, it might be Sasquatch or it could be a flying saucer, you know," local historian Feliks Banel told NBC News last month, more than a little tongue in cheek.

That the real culprit is something as mundane as a steel pipe buried only in the past decade might disappoint some, but not the Transportation Department.

“Our focus right now is on resuming tunneling as quickly and safely as possible,” Preedy said.