The paper examines, by means of highly resolved large-eddy simulations, the fluid mechanic behaviour of an incompressible, turbulent flow separating from an axisymmetric, hill-shaped obstacle. The hill is subjected to a turbulent flat-plate boundary layer of thickness half of the hill height, developing within a large duct in which the hill is placed. The Reynolds number based on the hill height and the free-stream velocity is 130000, and the momentum-thickness Reynolds number of the boundary layer is 7300. Extensive and detailed experimental data are available for these same conditions, and these are compared with the simulations. Two entirely independent simulations are reported, undertaken by different groups with different codes and different grids. They are shown to agree closely in most respects, and any differences are carefully identified and discussed. Mean-flow properties, pressure distributions and turbulence characteristics are reported and compared with the experimental data, with particular emphasis being placed on the separation region that is formed in the leeward of the hill and on the near wake. The connection between the wall topology of the mean flow and the secondary motions in the wake is discussed in detail. Finally, some aspects of the unsteady flow are analysed, including the unsteadiness of the separation process and structural features and coherent motions in the wake.