Ah, the Big Dig!... the most complex and expensive highway project ever attempted in the USA. Basically, it is a huge underground turnpike system spanning all over Boston that is designed to reduce vehicle traffic on the surface, cull traffic jams, and replace the aging raised highways. It may work, if they ever finish it (it's been under construction since 1990, and has cost dozens of billions of dollars).

More info: The Boston Central Artery Tunnel Project (“Big Dig”)