According to Boston.com, the Ted Williams Tunnel, at 90 feet below the surface, is the deepest in North America.  The tunnel was completed in 1995.  It was the first part of the Big Dig to be completed, and the last part of the Big Dig to be completed on schedule and within budget.  It is the subject of the first episode of the Discover Channel’s “Engineering Marvels.”  Watch it now: