K3 surfaces have been studied from many points of view, but the positivity of
the cotangent bundle is not well understood. In this paper we explore the
surprisingly rich geometry of the projectivised cotangent bundle of a very
general polarised K3 surface $S$ of degree two. In particular, we describe the
geometry of a surface $D_S \subset \mathbb{P}(\Omega_S)$ that plays a similar
role to the surface of bitangents for a quartic in $\mathbb{P}^3$.