How deep is the Big Dig in Boston?

The deepest point of the underground highway is 120. That's beneath the Red Line subway tunnel at Dewey Square (Atlantic Avenue and Summer Street). The highest point is at State Street, where the highway passes over the Blue Line subway tunnel and the roof of the highway is the street above.



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WASHINGTON, July 10 — The ceiling collapsed in one of Boston's Big Dig tunnels a year ago, killing one woman, because builders used the wrong epoxy to hold the anchor bolts in place, the National Transportation Safety Board said Tuesday.

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Perhaps ironically, the Blue Line takes several points of pride from its proximity to water, which gives it its name. Local historians claim the tunnel that runs under the harbor as the first underwater subway tunnel in the United States.

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The Tremont Street subway in Boston's MBTA subway system is the oldest subway tunnel in North America and the third oldest still in use worldwide to exclusively use electric traction (after the City and South London Railway in 1890, and the Budapest Metro's Line 1 in 1896), opening on September 1, 1897.

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Later, non-commercial traffic was allowed to access the tunnel on weekends and holidays. In 2003, with the substantial completion of the I-90 portion of the Big Dig, the tunnel was opened to all traffic at all times. The tunnel is 8,448 feet (2,575 m) long, of which approximately 3,960 feet (1,210 m) are underwater.

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