The three-dimensional turbulence structure within a region of significant turbulent fluid entrainment activity of a Mach 2.49, axisymmetric base flow is investigated experimentally utilizing tomographic particle image velocimetry. This region of significant fluid entrainment is identified and discussed using a turbulent quadrant analysis performed on a large ensemble of stereoscopic particle image velocimetry data in a locally defined streamwise-normal coordinate system. Statistical evidence is provided, utilizing linear stochastic estimation (LSE), that the three-dimensional turbulence in this region that evolves from mean flow shearing-induced turbulence is dominated by counter-hairpin vortices. These structures exhibit a concentrated turbulent bursting of fluid along the junction of the head and legs of the structure. In order to better understand the instantaneous flow structure of turbulent entrainment/detrainment events in this region of the flow field, a practical method by which to compute volume flux across an arbitrary, instantaneous flow boundary was developed, and the methodology is discussed. Utilizing this method in conjunction with LSE revealed that turbulent detrainment activity is statistically dominated by these counter-hairpin vortices, which convect strongly in the negative radial direction across the boundary separating the shear layer from the recirculation region. These structures are demonstrated to induce a large-scale fluid motion, where high-momentum fluid is extracted from the shear layer and is injected into the recirculation region. These findings identify a significant turbulent mechanism that is directly influential towards the resulting average base pressure, and provides significant groundwork upon which to base future research in the development of effective flow control methods for blunt-faced axisymmetric bodies in supersonic flight.