Recently, Evrard et al. (2016, Journal of Fluids and Structures, 61) achieved drag reduction by almost 9 % by means of a base cavity on a three-dimensional bluff body, the squareback Ahmed body. The authors associate drag reduction with the suppression of the static asymmetric modes of the wake identified by Grandemange et al. (2013, Journal of Fluid Mechanics, 722) leading to its symmetrization. The beneficial effect of a base cavity on the drag has been known for decades on axisymmetric bluff bodies in the context of aerospace engineering (Morel, 1979, Aeronautical Quarterly, 30) but the phenomenon has not been fully elucidated yet. The present work aims at showing experimentally that the decrease of the asymmetry of the near wake flow is responsible for drag reduction regardless of the ground clearance. With this aim in mind, we do two parametric studies of the ground clearance of a squareback Ahmed body in an industrial wind-tunnel; one without and one with a base cavity. We want to compare the two bifurcations of the wake operated by the ground clearance (Grandemange et al., 2013, Physics of Fluids, 25) depending on the rear geometry. Far enough from the ground i.e. after the bifurcation, the modes indeed disappear in agreement with the work of Evrard et al. the flow is symmetrized and the base pressure increases by 24 %. In the vicinity of the ground however, two new results are reported in this paper. The modes governing the wake are not the same as at higher ground clearances but are rather vertical modes due to the presence of the ground. Besides, even if the cavity does not fully symmetrize the flow in this case, its asymmetry is reduced. Consequently, an important drag reduction of the same order of magnitude as in the far ground regime is observed (7 % associated with a base pressure increase by 20 %). A six-components aerodynamic balance and twenty-one instantaneous base pressure measurements are used to record the data at a high sampling rate to study the dynamics of the changes.