The cotangent bundle of K3 surfaces of degree two

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 $D_S\subset \mathbb{P}({\mathrm{\Omega }}_S)$ that plays a similar role to the surface of bitangents for a quartic in ${\mathbb{P}}^3$.