Despite the recent interest in the novel and unique properties of two-dimensional materials, the melting behaviour of the simplest two-dimensional material, hard disks, is still intensely disputed. Crucially, until now – more than half a century after the first simulations of hard-disk freezing were published – no reliable experimental observations of the phase diagram of two-dimensional hard spheres have been reported. In this talk, I will describe how we determine the full phase behaviour of quasi-two-dimensional colloidal hard spheres [1, 2] by considering a tilted monolayer of particles in sedimentation-diffusion equilibrium [3]. In particular, we measure the equation of state from the density profiles and use time-dependent and height-resolved correlation functions to identify the liquid, hexatic and crystal phases. We find that the liquid-hexatic transition is first order and that the hexatic-crystal transition is continuous. Furthermore, we directly measure the width of the liquid-hexatic coexistence gap from the fluctuations of the corresponding interface, and thereby experimentally establish the full phase behaviour of hard disks [3]. [1] A.L. Thorneywork, R. Roth, D.G.A.L. Aarts and R.P.A. Dullens, J. Chem. Phys. 140, 161106 (2014). [2] A.L. Thorneywork, R.E. Rozas, R.P.A. Dullens and J. Horbach, Phys. Rev. Lett. 115, 268301 (2015). [3] A.L. Thorneywork, J.L. Abbott, D.G.A.L. Aarts and R.P.A. Dullens, Phys. Rev. Lett. 118, 158001 (2017).