Bertha, the SR 99 tunneling machine, is nearly a third of the way along its path to create a new underground roadway in Seattle. On Thursday it stopped for the latest round of scheduled maintenance — another challenge on the list of many, as workers must perform inspections and make repairs in hyperbaric conditions.

The Washington State Department of Transportation says Bertha has tunneled 3,088 of the planned 9,270 feet of the eventual tunnel. Bertha traveled 774 feet in June and more than 1,500 feet since its last pit stop.

The machine has built 466 of the 1,426 rings that comprise the tunnel wall.

The front end of Bertha’s cutterhead is now located approximately 120 feet beneath Spring Street, near Post Avenue. WSDOT revealed how Seattle Tunnel Partners workers will descend below ground to inspect and replace cutting tools as needed on the $80 million boring machine.

According to WSDOT, workers use compressed air and a type of clay, known as bentonite, to create an air bubble that allows them to safely work in the area behind the cutterhead. The area would otherwise be filled by soil and water. Crews must first adjust their bodies to air pressure in the underground environment by spending approximately an hour inside specialized pressure chambers within the machine that help their bodies adapt to hyperbaric conditions.

WSDOT said “it’s the same process scuba divers go through during the course of an underwater dive, but STP’s workers won’t need diving gear.” Graphics on the Alaskan Way Viaduct Replacement Program website illustrate what goes into creating the safe working environment:

Dense and stable soil conditions make the current position of Bertha more suitable for this maintenance stop. As it next moves into zone 4 — out of a total 10 — Bertha will encounter soils “that spent thousands of years sitting under the weight of mile-high glaciers,” WSDOT said. The route will take the machine past the foundation of the northbound SR 99 off-ramp to Seneca Street.