The Cathedral of Learning is part of the University of Pittsburgh and used as classrooms and study lounges (lucky undergrads!!). The architecture is charming and beautiful, particularly the ground floor. There are also Nationality Rooms on the 1st and 3rd floors (maybe more but there's no visitors guide). Most of them were holding lectures when we visited on a weekday morning and the peepholes were too small for us to view the intricate details of each room. I would love to come back and explore all 29 Nationality Rooms when class is not in session.
Note: their elevators are also very slow and only the "old" ones go up to the 3rd floor but they have stairwells around the corners.