You'll never think about Bryant Park the same way ever again. Promise.
The Bryant Park Stack Extension (or BPSE, pronounced "BipSee") is an enormous underground book vault connected to the New York Public Library's Humanities and Social Sciences Library (you know, the famous one with the lions out front) on 42nd street and 5th avenue in Manhattan, and contains the bulk of the library's film and video collections. The stack extension stretches under the entire length of Bryant Park clear from the library's back facade to 6th avenue, and houses 1.5 million volumes in subterranean movable shelving units as well as an administrative center. BPSE is climate controlled, with a standard temperature of 68°F and 50% relative humidity.
Construction of the extension was begun sometime prior to 1990 in 1988 (props to another E2 library mofo) as a way to keep the library system's humanities and social science collections housed under one roof - as it was the collection was split between the Humanities library on 42nd street and the Science, Industries and Business Library (or SIBL) a newer facility on 34th street that had shelf space to spare.
The book delivery system that runs through the rest of the library stretches down to these stacks as well, though if requesting books from these stacks you should be prepared to wait as long as 45 minutes to get your hands on them.
If you're standing by Bryant Park's fountain near 6th avenue sometime, look around by your feet - you should see two hatches let into the ground. Those hatches are the stack extension's emergency escape route should anything go wrong underground. There are also two more exits, one on the north side of the park and one on the south.
Climate controlled or not, BPSE is creepy. The marble and granite that is prominent throughout the rest of the building stops right before the long, steep ramp down into the stack extension's coffers. It's cold, industrial and efficient. The lighting is extremely fluorescent and hard on the eyes and the air is a bit stale. I never thought I could be a claustrophobic until I spent 2 summer weeks in BPSE.
Sources:
www.nypl.org/research/chss/about/quickfacts.html
personal experience (shudder)