How Deep Is The Deepest Tunnel In Boston, Massachusetts?

Whenever we're visiting the eastern part of Massachusetts, my kids prefer that I drive through Boston on the way to and from The Berkshires. They love the tunnels, and it's always nice to catch a glimpse of Fenway Park as well.

I've always wondered just how far below sea level the tunnels actually go - so I looked it up.

The Deepest Tunnel In Boston, Massachusetts

The 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.

It’s part of Interstate 93 and was built in 1959 as part of the Central Artery project, later upgraded during the Big Dig. This tunnel goes 120 feet underground, passing beneath the Red Line subway tunnel. It’s a cut-and-cover tunnel, meaning workers dug a trench, built the tunnel, and covered it up. -mass.gov

Are all tunnels in Massachusetts under water?

Not every tunnel in Massachusetts is underwater. 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. The Thomas P. O’Neill Jr. Tunnel, also part of the Big Dig, is close by but shallower, at about 50–70 feet deep.

How much pressure is on top of the tunnels?

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.

Fun Facts

• Massachusetts tunnels like the Ted Williams use special seals to keep water out, and some pump out thousands of gallons of saltwater monthly to stay dry!