From a NYPL News Release:
As described in recent public meetings, among the next steps for the renovation of the 42nd Street library is preparing the research collection that resides in the stacks underneath the Rose Main Reading Room. Specifically, we need to make sure that the books and other materials in the stacks are properly preserved and easily accessible in the months ahead as we undertake some pre-construction work (including work on various mechanical equipment).
We have more than three million books and other materials in the above-ground stacks so, understandably, the plan is complex. The first step was barcoding, so that the core of the Library’s research collection can now be discovered online by patrons, and be accessed more quickly by librarians. Next, we moved materials that have been requested by researchers in recent years to the existing, modern stacks underneath Bryant Park, where these materials are protected from sunlight in a space with proper humidity and temperature controls. Thus, we have retained onsite the materials most requested by researchers.
As we announced in fall 2012, we are planning to build out an additional floor of storage underneath Bryant Park. In the meantime, the remaining materials have been moved to off-site storage (in Patterson, New York, in a facility used by other world-class institutions), from which we can deliver requested materials within 24 hours, six days a week.