What is the smell at the 34th St/Penn Station Subway Station?

After enjoying a wonderful Thanksgiving, I returned back to the city from Long Island last night and again ran into a casual acquaintance of mine: that smell in the 34th Street Subway Station.

As soon as you enter the turnstile to the 1/2/3 red line, you enter a small tunnel that connects the uptown and downtown platforms of the express and local trains. As you walk to the local 1 train uptown platform, you are greeted by a horrible smell. It has been there for quite some time (over a year at least) and doesn't seem to be getting stronger or weaker. It's just a constant funk that hangs in the air and is absolutely nauseating.

I have no idea what it is or what is causing it. All I do know is that each time I take the subway at Penn Station, I have to suffer through it...