SEATTLE, WA - Bertha, the SR 99 tunnel boring machine, will complete drilling underneath Seattle sometime on Monday and will emerge from underground on Tuesday, according to the Washington state Department of Transportation.

Bertha - the largest boring machine of its kind on Earth - was about 30 feet from the surface on Monday afternoon. WSDOT said that Bertha would reach the edge of a concrete wall at the tunnel's end on Monday, and then would break through in several stages beginning early Tuesday.

"Crews will spend this evening making final preparations. Then, early tomorrow, the massive machine will begin mining through the wall and into the pit where it will be disassembled," WSDOT wrote in a blog post on Monday afternoon. "Machine operators will proceed slowly through the wall, so it may take several hours for the cutterhead to emerge."