The Stacks (also known as the Underlibrary or the Catalogue) were the repository of special items obtained or created by Faction Paradox.
They existed in the Eleven-Day Empire. Originally in Victoria Tower, Father Stendec came up with the idea of moving them to the underground stations and tunnels. Secured in deadend timelines using Lobachevsky's theories of non-Euclidean geometry, they could only be accessed by complex rituals. (PROSE: The Book of the War)