When you think “New York” and “library” there’s really only one that comes to mind: the grand main branch on Fifth Avenue facing Bryant Park. It’s gorgeous, it’s iconic, they have just about every book you can imagine (even if they’ve started trying to move a bunch of titles off-site); in other words perfect. The main Brooklyn branch on Grand Army Plaza is no slouch, either, either in architectural grandness or selection.

But there’s another library worth mentioning, and that’s the one underneath the Surrogate’s Court building in downtown Manhattan that nobody knows about: the City Hall Library. From last Tuesday’s New York Times:

Below the library are the cavernous storerooms and vaults that contain some of the maps, books, photographs and other items that are part of the Municipal Archives. They document the city’s government and leadership dating back to the unification of the boroughs into New York City in 1898, and back to the first mayor of the city, Thomas Willett, in 1665.

The history of the city is celebrated in old sepia photographs, wall-size topographical maps and reproduction manuscripts on display within the library and in a visitor center next to the library.

Yet there is not a hint of any of this on the granite exterior of the imposing Beaux-Arts building at 31 Chambers Street, behind City Hall….The librarians say the courthouse’s status as a designated landmark means that they are not allowed to hang a sign on the building’s exterior.

If you can’t get there, though, no worry: Much of the Municipal Archives’ photographic holdings were digitized and put online last year—the interest was so immediate that it crashed the server. It’s an astonishing collection of historical imagery, which you can see here. Many of the old glass-plate negatives that the NYPD shot of crime scenes back in the early 20th century were used by Luc Sante in his ghostly book, Evidence.