Second only to The Nutcracker, Igor Stravinsky's Le Sacre du Printemps is perhaps the most widely known and immediately recognizable ballet score among both dance and music audiences. How then, can a work with over two-hundred different choreographies and one-hundred commercial recordings be a lost ballet? The reality is that all ballets are, in some form, lost ballets, a byproduct of the ephemeral nature of the medium, where the dance exists in the moment it is performed and ceases to exist upon its conclusion, or upon the passing of its creator. When we hear a musical excerpt from The Nutcracker, Le Sacre, Swan Lake, or any score composed for dance and identify it as a ballet, we are referring to only one-half of an intended whole. Dances are often lost in the collective memories of both dancers and audiences, whereas the contents of the musical score can be experienced repeatedly and in many cases, on-demand. As we imagine the physical ballet through the aural experience of a musical score, we should consider the ballet as lost since it can only be recalled in our imagination. Using Le Sacre du Printemps as an example, Matijas Mecca will discuss the challenges in preserving choreographies and performance practices in dance and will address the challenges one can face in knowing a dance.