The digitization of library resources and services have opened up physical informational spaces to new dimensions by allowing users to access a wealth of documents in ways that di昀昀er from browsing bookshelves traditionally organized according to the "tree of knowledge". How do readers of digital library orient themselves within big corpora? What landmarks do they use to navigate masses of digital documents? Taking Gallica as a case study-the digital heritage platform of the French national library-, this paper presents an experimental research on the navigation practices of its users. Using methods from topological data analysis, we inferred from Gallica's server logs an informational space as it is roamed by readers. Coupled with user interviews, this mixed-methods study allowed us to identify a set of "regimes of navigation" characterizing how readers deploy various strategies to browse the digital library's corpus. From directed search to wandering to crawling, these regimes answer di昀昀erent needs and show that a single corpus can, in turns, be apprehended as a heritage collection, a database, a set of documents, and a mass of information.