What you get when you cross a highway project with an urban renewal project. The major components are:
The Ted Williams Tunnel, which extends the Mass Pike (Interstate 90) east to Logan Airport. This will also connect the Southeast Expressway to Logan; the Boston end of the Sumner Tunnel (SB) and Callahan Tunnel (NB) (which carry MA 1A) will be reworked for direct access only to the north.
The Central Artery, which is being rebuilt from Andrew Square to Charlestown, including a tunnel from Chinatown (where one already exists and will become the SB lanes) to the Charles River and a new cable-stayed bridge over the Charles.
The Leverett Circle Connector, which is a new bridge over the Charles River with direct access to US 1 to the Tobin Bridge and Interstate 93. This also includes a tunnel under Leverett Circle for eastbound traffic, which will complete Soldiers Field Road and Storrow Drive as a freeway from Harvard to Interstate 93. There will still be a traffic light for traffic from Storrow Drive to Interstate 93 southbound though :(
The Charlestown tunnel on US 1, which was the first thing completed in the Big Dig. New loop ramps were built to Interstate 93.

Cost overruns have plagued the project, which have been caused by adding the Ted Williams Tunnel and mistakes in measuring the geography.