Deep beneath Seattle, something has brought the world's biggest tunnel boring machine to an abrupt halt. The tunnel is currently just 1,000 feet but planned to be almost 10,000 feet long.

"We’re being really cautious. We want to make sure we don't damage this $80 million machine,” said Washington State Department of Transportation’s KaDeena Yerkan.

The $80 million machine, nicknamed Big Bertha, stands five stories tall. It arrived in Seattle earlier this year with much fanfare and high hopes.

Gregory Hauser is the deputy project manager for the tunnel which is planned to stretch 1.7 miles under downtown Seattle, carrying a double-deck highway. It will replace the Alaskan Way Viaduct, a raised concrete freeway built across Seattle's waterfront in the 1950's.

“Absolutely it's the way to go - every city should have a big tunnel underneath it,” said Hauser. “Alaskan Way Viaduct, which is old and ugly and dirty, and it's gonna fall down if the earth moves and it shakes any more, it's gonna fall.”

Big Bertha doesn't just drill - it installs concrete wall panels as it goes, leaving a finished tunnel behind.

“This is a work of art,” said Hauser. “This is absolutely a work of art and it's going to be beautiful when it's finished.”

The $3 billion tunnel is scheduled to carry traffic by the end of 2015 but engineers always knew they could run into obstacles.

“We’ve got a general idea of how many boulders there might be and what size they might be,” said Hauser. “But, there's no guarantee that there couldn't be a 20-footer sitting in the way.”

The project director says it could take two weeks to figure out what is in the way and how to remove it. Wells are being drilled to lower the pressure of ground water in front of the machine and divers may be sent in to have a look.

Big Bertha is named after Bertha Knight Landes, who was elected as Seattle's first female mayor in the 1920s.