Boston's "Big Dig" project took decades to complete, and now estimates peg its cost at $24 billion, according to the Boston Globe.

The project moved the city's central highway artery underground, helping to alleviate traffic and create the Rose Kennedy Greenway. But digging the tunnel cost over $14 billion, plus another $7 billion in interest on the project's bond, which expires in 2038, according to the Globe.

Add in supplemental transportation projects, including the implementation of the Silver Line rapid transit service, upgrades to the MBTA's blue line and the in-progress Green Line Extension project, and the Big Dig's true cost equals $24 billion. (Read the full Globe Report: True cost of Big Dig exceeds $24 billion with interest).

That number could increase slightly, depending on the final cost of the Green Line Extension.  The project, which will extend the green line northbound through Somerville with a final station in Medford, has been prevously estimated to cost about $1 billion. But its still in its relative infancy -- it isn't expected to be completed until 2020.