Our rediscovery of a seventeenth-century postmasters' trunk in the Museum voor Communicatie in The Hague, containing some twenty-six hundred undelivered letters mostly sent from France, offers the opportunity to think from the ground up about what constitutes an archive and how to approach it. We argue that understanding the process of loss, destruction, and survival of collections is a crucial exercise for historians. Practicing this “archaeology of the archive” makes us keenly aware that the questions we ask are often dictated by the genesis and structure of the archive. Although document survival is often the result of intentional safekeeping, in other cases it can be attributed to sheer accident. Addressing questions of materiality, mobility, and preservation, this article explores the notion of the “accidental archive” to consider what best practices should be developed to ensure responsible access to this unique collection.