BOSTON -- People will have their first chance to see the Big Dig tunnel, as a portion of the tunnel will be open for tours.

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Motorists will be driving underground by December when they travel Interstate 93 northbound, but walking tours will give taxpayers their first chance to see the tunnel, part of the $14 billion Big Dig project.

The tunnel is one of the largest underground highways ever designed. People taking the tour will travel 125 feet below the surface, and officials recommend they bring comfortable walking shoes and a camera.

The tour takes place between noon to 5 p.m. Sunday and is free. The walk begins at the Tunnel Project headquarters at 185 Kneeland St.