ADDED on 10/26/2015: Here's a link to a recent article about the caverns that store the special collections, and more: http://www.minnpost.com/stroll/2015/10/subterranean-caverns-protect-us-andersen-library-collections
Andersen Library houses special collections, and includes the caverns carved out of limestone about 10-15 years ago to house various archives.
My Children's Literature course met here last week, because it is the home of the Children's Literature Research Collection. Part of that collection is the Kerlan Collection, which includes manuscripts, graphics, and correspondence representing hundreds of authors and illustrators.
At that class, I learned about the "First Friday" lectures, so I went over today to hear two speakers. The first, representing the Tretter Collection in Gay, Lesbian, Bisexual and Transgender Studies, talked about gay marriage, and surprised us with an article on the subject from 1950.
The second speaker represented the Wangensteen Historical Library of Biology and Medicine, and talked about patent medicines; these nostrums rarely helped the conditions they were touted for, but probably provided a certain relief through their common ingredients, alcohol and opium.
When my class met here, I got the impression that after the First Friday lectures there would be an opportunity to tour the storage areas (which have a formal name that I have forgotten, but are known informally as The Bat Cave). I have an ailing kitty at home today, so I didn't stick around. But next month's speakers will represent the Immigration History Research Center and the Kirschner Cookbook Collection, and I hope to get a tour then. I'll update this post, or add a comment, if I get a tour.
The building is worth visiting on any given day for the displays created by the various collections.