Harvard’s flagship library, Widener, is an imposing granite cube built quite literally as shrine to the book. A central alcove cuts through the stacks to show off a prized relic: an original Gutenberg bible. But this is not the heart of Harvard’s libraries. No, that would be its cold storage site, an anonymous concrete building few students or even faculty know about.

The Harvard Depository, some 30 miles from the Cambridge campus, better resembles an Amazon warehouse than a library. The 200,000 square foot facility houses the vast majority of Harvard Library’s collection—some 9 million books, films, LPs, magnetic tapes, and pamphlets sorted not by the Dewey decimal system but by size.

A fascinating new interactive documentary, Cold Storage, glimpses inside this little-known world.

READ MORE: A Glimpse Inside the Hidden Vault Where Harvard Keeps Millions of Books | Gizmodo