Beneath: A documentary about NYC tunnel dwellers by Katja Knoll
If anybody is curious, this ramp to this portion of the tunnel is still open and very easily accessible. Not really a secret at all, but if you've never been...
PBS: The Race Underground
I forget if I've posted about this. Maybe I have. But in case you're hearing about this for the first time, here's a great documentary film from PBS' American Experience about the race to build the nation's first underground subway in Boston and New York.
The film is based on the book The Race Underground b Doug Most.