Creator's Description: This article explores the myths of massive root tantras of one hundred thousand or more stanzas, which are found in the literature surrounding most of the major tantric Buddhist traditions. These myths are curiously persistent, and arguably constitute a basis of authority for the tantric traditions. By portraying the tantras as revelations of much more extensive scriptural collections that are ancient or eternal, these myths legitimated the production of new Buddhist scriptures. They also impacted the formation of the canons of tantras, such as the Bka' 'gyur, and problematized their closure, insofar as Buddhist communities accepted the possibility of the continued revelation of the tantras.