Butler 310
Butler 310 is a room in Butler Library that contains the library's remaining card catalogs. The catalogs were among the first to use the Dewey Decimal System, and were actually arranged by Melvil Dewey himself when he was chief librarian at Columbia. Although they hadn't even been updated since 1985 (as the library website notes, "major historical changes, such as the break-up of the Soviet bloc in 1991, are not reflected"[1]) the card catalogs were finally slated to be removed over the course of 2013-2014, and the room turned into a "Digital Humanities Center," whatever that means.
In addition to the card catalogs, Butler 310 contains some intriguing study nooks, as it's actually a two-tiered room with a partial "second story" mezzanine that wraps around most of the space.