The Deepest Tunnel In Boston, MassachusettsThe Dewey Square Tunnel is the deepest tunnel in Massachusetts, located in Boston under the financial district near Dewey Square, at Atlantic Avenue and Summer Street. This tunnel goes 120 feet underground, passing beneath the Red Line subway tunnel. The Dewey Square Tunnel is on land, but the nearby Ted Williams Tunnel runs under Boston Harbor, reaching 90 feet deep, making it North America’s deepest underwater road connection. At 120 feet, the Dewey Square Tunnel handles about 5,200 pounds per square foot of pressure from the soil and buildings above—like 20 cars stacked on one square foot! For comparison, the Ted Williams Tunnel faces extra pressure from water, about 4,000 pounds per square foot at its deepest point.