Excavation of the 3rd Avenue tunnel segment began with the ceremonial launch of the "Mighty Mole", a , tunnel boring machine (TBM), on March 6, 1987. The TBM, designed by Robbins Company of Kent, Washington, and built by Nicholson Manufacturing in Seattle, began digging the western tunnel from Union Station the following May. A second, identical "Mighty Mole" TBM began digging the parallel eastern tunnel on June 29. During tunnel boring under 3rd Avenue between Spring Street and Madison Street on October 21, a small earthflow damaged a water main and caused pavement on 3rd Avenue to drop , shutting down water in the nearby Seattle City Light and 1001 Fourth Avenue Plaza buildings. While repairing the broken water main, electricians working on damaged high-voltage cables caused a small power outage that affected eight downtown buildings on the night of October 28, but were able to restore power by the following morning. Work on the western tunnel was briefly interrupted in November, when the TBM hit an unexpectedly large pocket of loose sand under Madison Street that had to be stabilized with grout to prevent damage to the adjacent Seattle City Light building. Boring on both tunnels was stopped in early January 1988, when a pocket of wet sand was encountered before the planned 90-degree turn onto Pine Street. Metro and tunnel contractors Atkinson/Dillingham, who had scheduled tunnel excavation for completion in mid-January, closed 3rd Avenue between Pike and Pine streets and installed 40 drilled wells to remove water from the sand pocket in February. Digging resumed on the western tunnel on March 14, and the TBM reached Westlake station on April 9, completing the first of the two tunnels. The eastern tunnel was completed a month later on May 18, allowing for parts of the TBMs to be salvaged and the steel outer shells to be buried in the tunnel.