One-dimensional hybrid code simulations are presented which show that magnetic field depletions of variable length and depht can be maintained over long time in mirror-stable high-¿ plasmas under certain conditions while magnetic compressions cannot. This suggests that short-scale magnetic holes (MHs), frequently observed in space plasmas, may be a feature of an essentially isotropic rather than a mirror-unstable plasma. This suggestion is supported by additional simulations in anisotropic plasmas which reveal that localized magnetic depressions are not a typical feature of saturated states of the mirror instability.