The World’s Largest Tunnel-Boring Machine

Meet the 25,000-horsepower Bertha.
Courtesy Washington State Department of Transportation

After a 2001 earthquake severely damaged the double-decker section of Seattle’s Route 99 highway, state and city officials decided to move the main thoroughfare underground. In July, they unleashed Bertha, the world’s largest-diameter tunnel-boring machine. Built in Japan and reassembled in the U.S., Bertha is digging the 1.7-mile tunnel by advancing at an average of 35 feet a day. As its 57.5-foot-wide head spins, 4-by-6-inch drill bits tear into the ground, dislodging small rocks and breaking up the soil. A set of spinning gears on the head crumbles large boulders. Scrapers then knock the dirt onto a conveyor belt that brings the haul to the surface. Bertha even has its own electric power source, so its 25,000-horsepower engine won’t disrupt the local grid. Seattle’s new double-decker highway is scheduled to open in 2015.

_This article originally appeared in the November 2013 issue of _Popular Science.