Mount Baker Ridge Tunnel: I-90's Giant Bore Under Seattle

Just before I-90 reaches Lake Washington, it ducks under the Mount Baker neighborhood of Seattle. The Mount Baker Ridge Tunnel is really a group of tubes. Together they carry eight lanes of freeway, a separate path for people on foot and bike, and two tracks for the Link light rail 2 Line. [1]

The first tunnels opened in 1940. They were twin bores, each 29 feet across, linking the new floating bridge to Rainier Avenue South as part of US Highway 10. Each tube held two lanes. In the 1960s a signal system made the middle lanes reversible for rush hour. At peak times, one tunnel carried two lanes going one way while the other carried traffic in both directions, split only by dashed yellow lines. [1]

When the highway became part of Interstate 90 and was expanded, crews built a new tunnel just north of the old ones. Work began in 1983, digging finished in 1986, and the tunnel opened in June 1989. It is 63 feet in diameter. Because it was bored through clay, it is the world's largest-diameter soft-earth tunnel. It was built in three levels: bus and carpool lanes on the bottom, general traffic in the middle, and cyclists and walkers on top. That top level, the I-90 Trail tunnel, is 1,330 feet long. [1]

The old and new tubes now work as a set. Eastbound traffic uses the 1940 tunnels and the Lacey V. Murrow Bridge. Westbound traffic uses the 1989 tunnel and the Homer M. Hadley Bridge. The lower deck once held reversible express lanes. They closed in 2017 to make room for light rail to the Eastside, which opened in March 2026. [1]

The tunnel was listed on the National Register of Historic Places in 1982. Its east portals carry murals by artist James FitzGerald called Portal of the North Pacific. You see them as you leave the tunnel heading east toward the lake. The official length is 1,440 feet, but it feels closer to 3,300 feet from behind the wheel, because a concrete lid extends the covered section to the west. [1]