The New York Public Library holds more than 16 million volumes, making it the 4th largest library in the United States. Many of those volumes are stored in the Milstein stacks, two levels of the library directly under Bryant Park. To maximize the storage capacity of these facilities and allow for patrons in the main library above ground to request volumes on site, the NYPL turns to some interesting technology. Gothamist recently toured the facility, showing off the inner workings of the knowledge vaults ordinarily reserved for library staff.