(Newser) – A giant drill boring beneath a section of Washington's State Route 99 has ground to a halt after hitting a mysterious clog some 60 feet underground. The five-story tunnel drill, Bertha, slowed to a crawl Saturday about 1,000 feet, or one-eighth of the way, into its dig beneath Seattle's Alaskan Way Viaduct, where a 1.7-mile replacement tunnel is supposed to open in 2015. "We don't know what the situation is," the project manager told the Seattle Post-Intelligencer, noting there are "thousands of guesses" as to what's blocking the way, including old city artifacts.

Bertha, the widest boring machine in the world, should be able to chew through concrete and boulders, but not steel. Drill rigs, which could drill over or near the blockage to figure out what it is, were put in place earlier this week, the Seattle Times reports, but it could be awhile before officials know what's behind the jam. Depending on what the team finds, tunnel workers may be forced to crawl 1,000 feet to the machine's face to get at the clog. "Ground penetrating" radar didn't find anything unusual before drilling began, the Times notes. (Read more Seattle stories.)

