The New York City 2nd Avenue Subway!
Initially running as the Q train from 63rd to 96th Street on the East Side of Manhattan, the 2nd Avenue Subway project is currently about 2/3 complete.
Eventually, when the line is completely finished, it will be renamed the T train and will run from Hanover Square in downtown Manhattan to 125th Street.
The estimated project cost? $4.4 billion!
This is an audio description of the project and some current pictures of the construction courtesy of WNYC.